Academic Commons Search Results
http://academiccommons.columbia.edu/catalog.rss?f%5Bdepartment_facet%5D%5B%5D=Physics&q=&rows=500&sort=record_creation_date+desc
Academic Commons Search Resultsen-usTopology of Reticulate Evolution
http://academiccommons.columbia.edu/catalog/ac:198973
Emmett, Kevin Josephhttp://dx.doi.org/10.7916/D8028RKGFri, 06 May 2016 18:23:54 +0000The standard representation of evolutionary relationships is a bifurcating tree. However, many types of genetic exchange, collectively referred to as reticulate evolution, involve processes that cannot be modeled as trees. Increasing genomic data has pointed to the prevalence of reticulate processes, particularly in microorganisms, and underscored the need for new approaches to capture and represent the scale and frequency of these events.
This thesis contains results from applying new techniques from applied and computational topology, under the heading topological data analysis, to the problem of characterizing reticulate evolution in molecular sequence data. First, we develop approaches for analyzing sequence data using topology. We propose new topological constructions specific to molecular sequence data that generalize standard constructions such as Vietoris-Rips. We draw on previous work in phylogenetic networks and use homology to provide a quantitative measure of reticulate events. We develop methods for performing statistical inference using topological summary statistics.
Next, we apply our approach to several types of molecular sequence data. First, we examine the mosaic genome structure in phages. We recover inconsistencies in existing morphology-based taxonomies, use a network approach to construct a genome-based representation of phage relationships, and identify conserved gene families within phage populations. Second, we study influenza, a common human pathogen. We capture widespread patterns of reassortment, including nonrandom cosegregation of segments and barriers to subtype mixing. In contrast to traditional influenza studies, which focus on the phylogenetic branching patterns of only the two surface-marker proteins, we use whole-genome data to represent influenza molecular relationships. Using this representation, we identify unexpected relationships between divergent influenza subtypes. Finally, we examine a set of pathogenic bacteria. We use two sources of data to measure rates of reticulation in both the core genome and the mobile genome across a range of species. Network approaches are used to represent the population of S. aureus and analyze the spread of antibiotic resistance genes. The presence of antibiotic resistance genes in the human microbiome is investigated.Evolution and development, Mathematicskje2109Physics, Biomedical InformaticsDissertationsQuantum phase transitions and local magnetism in Mott insulators: A local probe investigation using muons, neutrons, and photons
http://academiccommons.columbia.edu/catalog/ac:197987
Frandsen, Benjamin Allenhttp://dx.doi.org/10.7916/D8X06711Tue, 26 Apr 2016 15:34:42 +0000Mott insulators are materials in which strong correlations among the electrons induce an unconventional insulating state. Rich interplay between the structural, magnetic, and electronic degrees of freedom resulting from the electron correlation can lead to unusual complexity of Mott materials on the atomic scale, such as microscopically heterogeneous phases or local structural correlations that deviate significantly from the average structure. Such behavior must be studied by suitable experimental techniques, i.e. "local probes", that are sensitive to this local behavior rather than just the bulk, average properties. In this thesis, I will present results from our studies of multiple families of Mott insulators using two such local probes: muon spin relaxation (muSR), a probe of local magnetism; and pair distribution function (PDF) analysis of x-ray and neutron total scattering, a probe of local atomic structure. In addition, I will present the development of magnetic pair distribution function analysis, a novel method for studying local magnetic correlations that is highly complementary to the muSR and atomic PDF techniques.
We used muSR to study the phase transition from Mott insulator to metal in two archetypal Mott insulating systems: RENiO₃ (RE = rare earth element) and V₂O₃. In both of these systems, the Mott insulating state can be suppressed by tuning a nonthermal parameter, resulting in a "quantum" phase transition at zero temperature from the Mott insulating state to a metallic state. In RENiO₃, this occurs through variation of the rare-earth element in the chemical composition; in V₂O₃, through the application of hydrostatic pressure. Our results show that the metallic and Mott insulating states unexpectedly coexist in phase-separated regions across a large portion of parameter space near the Mott quantum phase transition and that the magnitude of the ordered antiferromagnetic moment remains constant across the phase diagram until it is abruptly destroyed at the quantum phase transition. Taken together, these findings point unambiguously to a first-order quantum phase transition in these systems. We also conducted x-ray and neutron PDF experiments, which suggest that the distinct atomic structures associated with the insulating and metallic phases similarly coexist near the quantum phase transition. These results have significant implications for our understanding of the Mott metal-insulator quantum phase transition in real materials.
The second part of this thesis centers on the derivation and development of the magnetic pair distribution function (mPDF) technique and its application to the antiferromagnetic Mott insulator MnO. The atomic PDF method involves Fourier transforming the x-ray or neutron total scattering intensity from reciprocal space into real space to directly reveal the local atomic correlations in a material, which may deviate significantly from the average crystallographic structure of that material. Likewise, the mPDF method involves Fourier transforming the magnetic neutron total scattering intensity to probe the local correlations of magnetic moments in the material, which may exist on short length scales even when the material has no long-range magnetic order. After deriving the fundamental mPDF equations and providing a proof-of-principle by recovering the known magnetic structure of antiferromagnetic MnO, we used this technique to investigate the short-range magnetic correlations that persist well into the paramagnetic phase of MnO. By combining the mPDF measurements with ab initio calculations of the spin-spin correlation function in paramagnetic MnO, we were able to quantitatively account for the observed mPDF. We also used the mPDF data to evaluate competing ab initio theories, thereby resolving some longstanding questions about the magnetic exchange interactions in MnO.Condensed matter physicsbaf2128PhysicsDissertationsAdvances in Lattice Quantum Chromodynamics
http://academiccommons.columbia.edu/catalog/ac:197677
McGlynn, Gregory Edwardhttp://dx.doi.org/10.7916/D8T72HD7Fri, 15 Apr 2016 18:29:00 +0000In this thesis we make four contributions to the state of the art in numerical lattice simulations of quantum chromodynamics (QCD). First, we present the most detailed investigation yet of the autocorrelations of topological observations in hybrid Monte Carlo simulations of QCD and of the effects of the boundary conditions on these autocorrelations. This results in a numerical criterion for deciding when open boundary conditions are useful for reducing these autocorrelations, which are a major barrier to reliable calculations at fine lattice spacings. Second, we develop a dislocation-enhancing determinant, and demonstrate that it reduces the autocorrelation time of the topological charge. This alleviates problems with slow topological tunneling at fine lattice spacings, enabling simulations on fine lattices to be completed with much less computational effort. Third, we show how to apply the recently developed zMöbius technique to hybrid Monte Carlo evolutions with domain wall fermions, achieving nearly a factor of two speedup in the the light quark determinant, the single most expensive part of the calculation. The dislocation-enhancing determinant and the zMöbius technique have enabled us to begin simulations of fine ensembles with four flavors of dynamical domain wall quarks. Finally, we show how to include the previously-neglected G1 operator in nonperturbative renormalization of the ∆S = 1 effective weak Hamiltonian on the lattice. This removes an important systematic error in lattice calculations of weak matrix elements, in particular the important K → ππ decay.Physicsgem2128PhysicsDissertationsInvestigation of Two-Dimensional Transition Metal Dichalcogenides by Optical and Scanning Tunneling Spectroscopy
http://academiccommons.columbia.edu/catalog/ac:197563
Rigosi, Albert Felixhttp://dx.doi.org/10.7916/D8WH2PXDWed, 13 Apr 2016 12:20:01 +0000The goal of this dissertation is not only to present works completed and projects initiated and accomplished, but to also attempt to teach some of the material to readers who have limited exposure to condensed matter. I will offer an introduction to two-dimensional transition metal dichalcogenide materials (2D TMDCs) and the mathematics required to understand the research conducted. Some effort will be given on explaining the experimental setups and preparations. Projects that required elaborate sample fabrication and the yielded results will be summarized. These results have heavy implications for the science behind bound electron-hole pairs, the effects of magnetic fields on such pairs, and extracting the useful optical properties from the material systems in which these pairs reside. Specialized fabrication techniques of samples for longer term projects that I led will also be presented, namely those of constructing heterostructures by stacking various 2D TMDCs for exploring the modulated properties of these novel arrangements. The latter portion of this dissertation will cover the nanoscopic dynamics of TMDC heterostructures. The Kramers-Kronig relations will be derived and discussed in detail.
Data and results regarding the electronic structure of these materials, their heterostructures, and their custom alloys measured via scanning tunneling microscopy will be presented. Coupled with the measured optical properties, significant numerical quantities that characterize these materials are extracted. There will be several appendices that offer some supplementary information and basic summaries about all the projects that were initiated.Physicsafr2117Physics, Electrical EngineeringDissertationsProbing Transition Metal Dichalcogenide Monolayers and Heterostructures by Optical Spectroscopy and Scanning Tunneling Spectroscopy
http://academiccommons.columbia.edu/catalog/ac:197551
Hill, Heather Mariehttp://dx.doi.org/10.7916/D88W3D85Mon, 11 Apr 2016 18:25:30 +0000Atomically thin two-dimensional materials, such as graphene and semiconductor transition metal dichalcogenides (TMDCs), exhibit remarkable and desirable optical and electronic properties. This dissertation focuses on the excitonic properties of monolayer TMDCs taken first in isolation and then in contact with another material. We begin with a study of the exciton binding energy in two monolayer TMDCs, WS₂ and MoS₂. We observe excited states of the exciton by two different optical spectroscopy techniques: reflectance contrast and photoluminescence excitation (PLE) spectroscopy. We fit a hydrogenic model to the energies associated with the excited states and infer a binding energy, which is an order of magnitude higher than the bulk material. In the second half of this work, we study two types of two-dimensional vertical heterostructures. First, we investigate heterostructures composed of monolayer WS₂ partially capped with graphene one to four layers thick. Using reflectance contrast to measure the spectral broadening of the excitonic features, we measure the decrease in the coherence lifetime of the exciton in WS₂ due to charge and energy transfer when in contact with graphene. We then compare our results with the exciton lifetime in MoS₂/WS₂ and MoSe₂/WSe₂ heterostructures. In TMDC/TMDC heterostructures, the decrease in exciton lifetime is twice that in WS₂/graphene heterostructures and due predominantly to charge transfer between the layers. Finally, we probe the band alignment in MoS₂/WS₂ heterostructures using scanning tunneling microscopy (STM) and spectroscopy (STS).We confirm the monolayer band gaps and the predicted type II band alignment in the heterostructure. Drawing from all the research presented, we arrive at a favorable conclusion about the viability of TMDC based devices.Physicshmh2131Physics, Electrical EngineeringDissertationsPerturbative and Nonperturbative Aspects of Jet Quenching in Near-Critical Quark-Gluon Plasmas
http://academiccommons.columbia.edu/catalog/ac:193924
Xu, Jiechenhttp://dx.doi.org/10.7916/D84Q7TSDWed, 27 Jan 2016 00:00:00 +0000In this thesis, we construct two QCD based energy loss models to perform quantitative analysis of jet quenching observables in ultra-relativistic nucleus-nucleus collisions at RHIC and the LHC. We first build up a perturbative QCD based CUJET2.0 jet flavor tomography model that couples the dynamical running coupling DGLV opacity series to bulk data constrained relativistic viscous hydrodynamic backgrounds. It solves the strong heavy quark energy loss puzzle at RHIC and explains the surprising transparency of the quark-gluon plasma (QGP) at the LHC. The observed azimuthal anisotropy of hard leading hadrons requires a path dependent jet-medium coupling in CUJET2.0 that implies physics of nonperturbative origin. To explore the nonperturbative chromo-electric and chromo-magnetic structure of the strongly-coupled QGP through jet probes, we build up a new CUJET3.0 framework that includes in CUJET2.0 both Polyakov loop suppressed semi-QGP chromo-electric charges and emergent chromo-magnetic monopoles in the critical transition regime. CUJET3.0 quantitatively describes the anisotropic hadron suppression at RHIC and the LHC. More significantly, it provides a robust connection between the long wavelength "perfect fluidity'' of the QGP and the short distance jet transport in the QGP. This framework paves the way for ``measuring'' both perturbative and nonperturbative properties of the QGP, and more importantly for probing color confinement through jet quenching.Physics, Nuclear physics, Particle physics, Quantum physics, Quark-gluon plasma, Scattering (Physics), Nuclear reactions, Nuclear physics, Color confinement (Nuclear physics), Quantum chromodynamicsjx2179PhysicsDissertationsHiggs and Particle Production in Nucleus-Nucleus Collisions
http://academiccommons.columbia.edu/catalog/ac:193251
Liu, Zhehttp://dx.doi.org/10.7916/D8M32VHGThu, 07 Jan 2016 00:00:00 +0000We apply a diagrammatic approach to study Higgs boson, a color-neutral heavy particle, production in nucleus-nucleus collisions in the saturation framework without quantum evolution. We assume the strong coupling constant much smaller than one. Due to the heavy mass and colorless nature of Higgs particle, final state interactions are absent in our calculation. In order to treat the two nuclei dynamically symmetric, we use the Coulomb gauge which gives the appropriate light cone gauge for each nucleus. To further eliminate initial state interactions we choose specific prescriptions in the light cone propagators. We start the calculation from only two nucleons in each nucleus and then demonstrate how to generalize the calculation to higher orders diagrammatically. We simplify the diagrams by the Slavnov-Taylor-Ward identities. The resulting cross section is factorized into a product of two Weizsäcker-Williams gluon distributions of the two nuclei when the transverse momentum of the produced scalar particle is around the saturation momentum. To our knowledge this is the first process where an exact analytic formula has been formed for a physical process, involving momenta on the order of the saturation momentum, in nucleus-nucleus collisions in the quasi-classical approximation. Since we have performed the calculation in an unconventional gauge choice, we further confirm our results in Feynman gauge where the Weizsäcker-Williams gluon distribution is interpreted as a transverse momentum broadening of a hard gluons traversing a nuclear medium. The transverse momentum factorization manifests itself in light cone gauge but not so clearly in Feynman gauge. In saturation physics there are two different unintegrated gluon distributions usually encountered in the literature: the Weizsäcker-Williams gluon distribution and the dipole gluon distribution. The first gluon distribution is constructed by solving classical Yang-Mills equation of motion in the McLerran-Venugopalan model, while the second gluon distribution is related to the dipole scattering amplitude. So far, the quantum structure of the dipole gluon distribution has not been thoroughly investigated. Applying the same diagrammatic techniques, we carry out a detail study of the quantum structure of the color dipole gluon distribution, and then compare it with that of the Weizsäcker-Williams gluon distribution.Theoretical physics, Higgs bosons, Theoretical physics, Gluons, Particles (Nuclear physics)zl2239PhysicsDissertationsInteractions and Disorder in Novel Condensed Matter Systems
http://academiccommons.columbia.edu/catalog/ac:189622
Lemonik, Yonah Shalomhttp://dx.doi.org/10.7916/D8T152Z3Tue, 22 Sep 2015 00:00:00 +0000Despite almost a century of exploration, we continue to discover new systems where quantum mechanics, strong interactions and disorder combine in novel ways. These systems test the capabilities of our strongest theoretical tools. In this thesis I discuss work on three of these systems: bilayer graphene, disordered conductors and cold atom systems. In bilayer graphene I show that the large number of degenerate bands leads to a plethora of possible spontaneous symmetry breaking ground state. In disordered conductors I discuss how quantum interference can lead to arbitrarily long lived responses, so called memory eects. I also consider whether a novel spontaneous symmetry breaking state can be created in cold atomic gasses using nonequilibrium perturbations.Condensed matter physicsysl2101PhysicsDissertationsKaon to Two Pion decay from Lattice QCD and CP violation
http://academiccommons.columbia.edu/catalog/ac:189610
Zhang, Daiqianhttp://dx.doi.org/10.7916/D8C828NSThu, 17 Sep 2015 00:00:00 +0000We report a direct lattice calculation of the K to ππ (ΔI=1/2) decay amplitude A₀ on a 32³×64 ensemble, with 2+1 flavor Möbius Domain Wall Fermion, with a⁻¹=1.379(9) GeV. This is a complete and physical calculation: chiral symmetry breaking is controlled by the Möbius Domain Wall formalism; pion and kaon masses are simulated at their near-physical values, mκ≅490 MeV and mπ≅140 MeV. G-parity boundary conditions are used to realize correct kinematics for the final two-pion state and give E_{ππ(I=0)}≅498 MeV, while keeping isospin symmetry; all 10 ΔS=1 operators are considered, each of which involve the notorious disconnected diagrams. With this setup, we are able to resolve, for the first time, the physical decay amplitudes Re(A₀) and Im(A₀) from 0. The Re(A₀) amplitude agrees with its experimental value, The result for Im(A₀) is used, in combination with the lattice calculated decay amplitude A₂, to compute Re(ϵ'/ϵ), which evaluates to 1.38(5.08)×10⁻⁴ and agrees at the 2σ level with the experimental value 1.66(0.23)×10⁻³. This is a major step towards understanding and testing CP violation in the standard model. Several measurement techniques are used to increase computational efficiency. We use all-to-all propagators to construct finite sized mesons, which have a better overlap with the meson ground state and reducing statistical noise from the vacuum graphs. This also saves matrix-inversion overhead when constructing mesons with different momenta. The other technical improvements include the mixed-precision conjugate gradient algorithm, and optimized fast Fourier transformation. We also discuss the cross-checks on the use of G-parity boundary conditions, and estimate several important systematic errors.Physicsdz2203PhysicsDissertationsMeasurements of Electron Antineutrino Disappearance in the Double Chooz Experiment
http://academiccommons.columbia.edu/catalog/ac:189517
Carr, Rachel Erinhttp://dx.doi.org/10.7916/D8W37VNNTue, 15 Sep 2015 00:00:00 +0000This thesis presents complementary measurements of the neutrino oscillation parameter sin²2θ₁₃ made by observing the electron antineutrino flux from two nuclear reactors in Chooz, France. Antineutrinos are identified through both products of the inverse beta decay interaction, bar νₑ + p → e⁺ + n, in a high-precision liquid scintillator detector located approximately 1 km from the reactors. The most sensitive signal channel involves neutron captures by a gadolinium dopant, while a search for neutron captures on hydrogen provides a high-statistics validation. In both channels, the value of sin²2θ₁₃ is revealed by an energy- and reactor power-dependent deficit of antineutrino-like events, compared to a reactor simulation. All analyses produce results consistent with one another and with findings of other experiments. These datasets also expose features of the antineutrino spectrum not predicted in reactor flux models. Data from the newly inaugurated near detector, anticipated in the final part of this thesis, holds unique potential to clarify these features, pursue signals from sterile neutrinos, and contribute to global knowledge of three-neutrino mixing.Particle physicsrec2126PhysicsDissertationsLocking the Advanced LIGO Gravitational Wave Detector: with a focus on the Arm Length Stabilization Technique
http://academiccommons.columbia.edu/catalog/ac:189457
Staley, Alexa Nitzanhttp://dx.doi.org/10.7916/D8X34WQ4Fri, 21 Aug 2015 00:00:00 +0000The Advanced LIGO gravitational wave detectors have recently achieved a new milestone. The two detector network is now operational and is being tuned for sensitivity. Currently, the state of the art detectors are the most sensitive ground-based interferometers to date and are closer than ever to the reality of a gravitational wave detection. For many years, there has been a worldwide effort to directly detect gravitational waves, a phenomena that was predicted in Einstein's theory of general relativity. A direct detection would further validate Einstein's theory, but more importantly would provide a novel approach to studying the universe and the elusive physics of gravity beyond Einstein's theory. However, none of this would be possible without the success of the arm length stabilization scheme. This recently demonstrated technique, which will be the focus of this thesis, is a critical step required to get the LIGO interferometers operational. This scheme is unique to the advanced generations of detectors and is extremely valuable for such a complex instrument. As part of my research, I characterized, modeled, and helped design this important technique. I was also a part of a small team that brought the LIGO Hanford interferometer to its operational point for the first time. For astrophysical reasons, the goal of Advanced LIGO's design is to measure a gravitational strain as small as 4x10⁻²⁴/rtHz, requiring a length resolution of approximately 10⁻¹⁹ m. This high sensitivity demands multiple optical cavities to enhance the response of the interferometer. The interferometer is a Michelson interferometer geometry consisting of two 4km arm cavities, whose differential length is measured by the phase change of a resonating infrared laser at the gravitational wave readout port. The Michelson interferometer is enhanced by Fabry-Perot arm cavities, a power recycling cavity, and a signal extraction cavity. The Fabry-Perot arm cavities effectively increase the arm lengths by two orders of magnitude. Meanwhile, the power recycling cavity is used to enhance the circulating power within the interferometer, and the signal extraction cavity is used to enhance the optical response at the gravitational-wave readout. Besides the increased design sensitivity of Advanced LIGO, a crucial requirement for a gravitational wave detection will be a high duty cycle. As an example, a worldwide Advanced LIGO network of five detectors, each with an 80% up-time, would only produce about 30% network up-time. A deterministic, robust, and fast sequence to transition the interferometer from an uncontrolled to a controlled state is mandatory. Advanced LIGO has five longitudinal degrees of freedom which must be controlled in order for the interferometer to be operational. However, all degrees of freedom are strongly coupled making this a traditionally challenging process. The state of the arm cavities can completely alter the state of the dual-recycled Michelson interferometer. Active feedback control is required to operate these instruments and keep the cavities locked on resonance. The optical response is highly non-linear until a good operating point is reached. The linear operating range is between 0.01% and 1% of a fringe for each degree of freedom. The resonance lock has to be achieved in all five degrees of freedom simultaneously, making the acquisition difficult. Furthermore, the cavity linewidth seen by the laser is only ~1Hz which is four orders of magnitude smaller than the linewdith of the free running laser. To mitigate several of these critical problems, a new arm length stabilization technique was introduced to the lock sequence. The arm length stabilization technique utilizes two additional green lasers that are brought into resonance in each arm cavity. This effectively decouples the arm cavities from the rest of the interferometer. While the main infrared beam is kept off resonance from the arm cavity, a modulation technique utilizing third harmonics locks the central dual-recycled Michelson interferometer. In the final step, both arm cavities are slowly tuned onto resonance, nominal sensors are used, and full lock is achieved. To ensure a high duty cycle for Advanced LIGO and confirm repeatability of the locking sequence, a detailed study and characterization of the arm length stabilization technique was conducted. A model of the scheme and a noise budget was developed. The model was beneficial while designing and implementing the scheme for the first time at the Advanced LIGO observatories. Meanwhile, the noise budget was critical to determine if this scheme would be viable in the lock process. Ultimately, the advent of the arm length stabilization to the lock process has been successful, a decisive milestone for future prospect of collecting meaningful astrophysical data. The technique has been implemented at both detectors and proven reliable and robust. Given the complexity of the interferometers, the success of this scheme to bring the detectors operational was a large accomplishment for the collaboration. With the technique's repeatable performance, efforts can be focused on tuning the interferometers sensitivity and achieving a first direct detection. This thesis begins with an introduction on the theory of general relativity and gravitational waves. Common astrophysical sources are described in Chapter 2. Chapter 3 begins with a description of the installed instrument. A discussion on the detector design sensitivity, limiting noise sources, and estimated detection rates is also given. At the end of Chapter 3, the complications of lock acquisition are highlighted. The arm length stabilization system was introduced to Advanced LIGO as a partial way to solve the difficulties of locking. Chapter 4 discusses the motivation for the use of this scheme and explains the methodology. A detailed discussion on the arm length stabilization model is given, along with the noise budget in Chapters 5 and 6 respectively. The full lock sequence is described in Chapter 7. The thesis concludes with the current status of the interferometers.Physicsans2161PhysicsDissertationsA study of Pulsar Wind Nebulae and non-thermal filaments with the NuSTAR observatory
http://academiccommons.columbia.edu/catalog/ac:188388
Nynka, Melania Christinahttp://dx.doi.org/10.7916/D88051TKMon, 06 Jul 2015 00:00:00 +0000NuSTAR, the first high-energy focusing X-ray telescope, has provided an unprecedented view of the universe above 10 keV. I first briefly describe the fabrication and calibration campaign of the NuSTAR optics at Columbia University. I then present two main areas of research with NuSTAR: the pulsar wind nebula (PWN) G21.5-0.9, and the investigation of several filamentary structures within 0.5 deg. of the Galactic Center. G21.5-0.9 is a well-studied PWN, and was observed by NuSTAR with ∼ 280 ks in the first months of its mission. I used both spectral and spatial image analysis of the emission to probe the validity of various magnetohydrodynamic models. Image deconvolution reveals the existence of non-thermal emission up to 20 keV, likely the supernova shell. Next I discuss three non-thermal filaments found near the Galactic Center. The Cannonball is a known high-velocity neutron star escaping the radio shell of Sgr A East with an extended radio and soft X-ray tail. NuSTAR extended its non-thermal spectrum to 30 keV and measured a magnetic field of ∼ 313−550μG. I analyze filament G359.97-0.038 by incorporating broad-band morphological and spectral data from radio (5.5 and 8.3 GHz) and X-ray data with NuSTAR data. I conclude that it is not a PWN but more likely the result of an interaction between the Sgr A East remnant and the nearby molecular cloud. Lastly I observe the filament G0.13-0.11, likely a PWN elongated by the ram pressure from the nearby Radio Arc.Physics, AstrophysicsPhysicsDissertationsTunable SU(4) Symmetry in Bilayer Graphene
http://academiccommons.columbia.edu/catalog/ac:188040
Maher, Patrick Thomashttp://dx.doi.org/10.7916/D8P26X8ZTue, 02 Jun 2015 00:00:00 +0000The charge carriers in bilayer graphene have both spin and valley degeneracy. Because of its unique electronic structure, valley symmetry in this material is connected to layer symmetry, which can be broken experimentally with a transverse electric field. Together with the coupling of spin to external magnetic fields, bilayer graphene makes for an experimental system with a tunable SU(4) symmetry space. This thesis describes experiments performed on ultra-high-quality dual-gated bilayer graphene heterostructures. In the quantum Hall regime, electric and magnetic fields can be used to probe the ordering of ground states, and to induce new orderings. In particular, at charge neutrality and high Zeeman splitting, we are able to tune the system to a ferromagnetic phase which exhibits a crossing of oppositely spin-polarized edge states, mimicking the quantum spin Hall effect. At higher fields in cleaner samples, we observe fractional quantum Hall states. These states also exhibit phase transitions, which show a clear but non-trivial connection with the phase transitions observed in integer quantum Hall states. In higher Landau levels, while the connection between layer and valley changes, we still observe phase transitions between different quantum Hall states by applying transverse displacement fields. We identify clear patterns in these phase transitions over a number of Landau levels. Finally, we present experiments on bilayer graphene with an aligned split-gate geometry. This system is predicted to support topologically-protected valley-polarized states. We discuss fabrication challenges and preliminary experimental results.Physicsptm2111PhysicsDissertationsSearch for a new resonance in the boosted di-Higgs to 4 bottom quarks final state at √s = 8 TeV using the ATLAS detector at the Large Hadron Collider
http://academiccommons.columbia.edu/catalog/ac:188025
Zhou, Leihttp://dx.doi.org/10.7916/D8959GPQThu, 28 May 2015 00:00:00 +0000This thesis presents a search for a new, heavy particle decaying to a pair of Higgs bosons in the 4 bottom quarks final state at √s= 8 TeV using the ATLAS detector at the Large Hadron Collider. The full data collected by ATLAS in 2012 at √s = 8 TeV is used, corresponding to a total luminosity of 19.5 fb -1. A novel technique, using smaller radius track jet to tag bottom quarks in combination with two large radius calorimeter jets to fully reconstruct boosted event topologies, significantly improves the sensitivity up to the mass scale of 2 TeV. In the absence of an excess, upper limits on the production cross section are set with 95% confidence level, using Kaluza-Klein gravitons in the bulk Randall-Sundrum model with coupling c ≡ k √M pl = 1.0 and 2.0 as benchmarks.Physicslz2279PhysicsDissertationsHolography, Locality and Symmetries of the Universe
http://academiccommons.columbia.edu/catalog/ac:188016
Xiao, Xiaohttp://dx.doi.org/10.7916/D8C53JZGThu, 28 May 2015 00:00:00 +0000It is an interesting question that, with a well tested duality between the quantum gravity in anti de Sitter space and a quantum field theory in one lower dimension, whether quantum gravity in a cosmological background has a well defined dual description. In large 1/N limit, this duality could be a correspondence between an approximately local gravity theory describing cosmology and a quantum field theory. In dS/CFT, the quantum field theory is a Euclidean CFT living at the conformal boundary of de Sitter space, in large N limit, we should expect the local observables in de Sitter cosmology be recovered from the CFT. We explicitly develop this construction for scalar fields and derive the operator map at lowest order of 1/N expansion. Having addressed the fundamental question of how local fields in de Sitter cosmology arise via holography, we focus on the theory of cosmological perturbations that is described in terms of local field theory. The curvature perturbations during inflation, which originated from quantum fluctuations of inflaton and which induced the CMB inhomogeneity we see today, as well as the large scale structure, can be described as Goldstone boson fields which nonlinearly realize a subset of general coordinate transformations as residual symmetries. This fact puts strong constraints on the behavior of the cosmological correlation functions, and a series of consistency relations constraining the soft limits of these correlation functions can be derived as Ward identities.Physicsxx2146PhysicsDissertationsIntegrated circuit-based electrochemical sensor for spatially resolved detection of redox-active metabolites in biofilms
http://academiccommons.columbia.edu/catalog/ac:185400
Bellin, Daniel L.; Sakhtah, Hassan; Thimot , Jordan; Emmett, Kevin ; Dietrich, Lars; Shepard, Kenneth L.http://dx.doi.org/10.7916/D8C24VBGTue, 31 Mar 2015 00:00:00 +0000Despite advances in monitoring spatiotemporal expression patterns of genes and proteins with fluorescent probes, direct detection of metabolites and small molecules remains challenging. A technique for spatially resolved detection of small molecules would benefit the study of redox-active metabolites that are produced by microbial biofilms and can affect their development. Here we present an integrated circuit-based electrochemical sensing platform featuring an array of working electrodes and parallel potentiostat channels. ‘Images’ over a 3.25 0.9 mm2 area can be captured with a diffusion-limited spatial resolution of 750 mm. We demonstrate that square wave voltammetry can be used to detect, identify and quantify (for concentrations as low as 2.6 mM) four distinct redox-active metabolites called phenazines. We characterize phenazine production in both wild-type and mutant Pseudomonas aeruginosa PA14 colony biofilms, and find correlations with fluorescent reporter imaging of phenazine biosynthetic gene expression.Molecular biology, Electrical engineering, Nanotechnology, Biophysicsdlb2149, hs2593, jat2176, kje2109, ld2444, kls30Physics, Electrical Engineering, Biological SciencesArticlesThe Classical and Quantum Aspects of the Detection of Gravitational Waves
http://academiccommons.columbia.edu/catalog/ac:184144
Factourovich, Maximhttp://dx.doi.org/10.7916/D83T9G2BFri, 06 Mar 2015 00:00:00 +0000Detection of gravitational waves has been one of the major undertakings of science for the past several decades. The elusive phenomenon first emerged as a natural consequence of the A. Einstein's Theory of General Relativity, but for many years was beyond the reach of the existing technological capabilities. Today, a radical effort is underway to take the measurement technology to a new, unprecedented level of sensitivity, in order to give a definite answer to one of the most fundamental aspects of our understanding of the Universe. The currently accepted detection scheme utilizes interference of near-infrared light inside a high-finesse Fabry-Perot cavity, and has achieved resolution on a scale of 10-21 as compared to the cavity length. At this scale, the signal becomes very sensitive to all kinds of unwanted inputs which include, but not limited to, the seismic activity, acoustic vibrations, thermal effects and radiation pressure noise. Moreover, the sensitivity requirements place it near the fundamental limit of quantum uncertainty which poses the ultimate barrier for lowering the detection threshold. Additionally, at the large kilometer-scale size of the installations, the signal propagation delays become significant enough to call for precise synchronization between the remote sensors and electronics within the main data collector. The need for this becomes even more evident considering a possibility of triangulation the otherwise non-directional signal, by unifying the data collected from different observatories spread around the globe. In this work, we first address the aspect of precise timing synchronization implemented in the US-based Advanced Laser-Interferometer Gravitational-wave Observatories (LIGO) located at Hanford, WA and Livingston, LA. The developed Advanced LIGO Timing System allows for synchronization of virtually unlimited number of devices to an accuracy of better than 1 microsecond, regardless of the distances involved. The machinery uses Field Programmable Gate Array (FPGA) logic at its core processing units. The FPGA chips are driven by oscillators synchronized to both, a Master atomic clock and the Global Positioning System (GPS) satellites for a precise calibration with redundancy. The timings signals are encoded in a pulse-modulated signal and distributed over the network via optical fibers. Additionally, we present a prototype device that allows overcoming the quantum sensitivity barrier without violating the Uncertainty Principle, also known as the Squeezer. We demonstrate the laser shotnoise reduction of up to 9 dB in a test setup, that eventually translated to a 25% increase in the detector sensitivity, upon injection of the squeezed light into the operational LIGO interferometer.Physicsmf2199PhysicsDissertationsPaul Drude’s Prediction of Nonreciprocal Mutual Inductance for Tesla Transformers
http://academiccommons.columbia.edu/catalog/ac:183558
McGuyer, Bart H.http://dx.doi.org/10.7916/D83F4NG3Thu, 26 Feb 2015 00:00:00 +0000Inductors, transmission lines, and Tesla transformers have been modeled with lumped-element equivalent circuits for over a century. In a well-known paper from 1904, Paul Drude predicts that the mutual inductance for an unloaded Tesla transformer should be nonreciprocal. This historical curiosity is mostly forgotten today, perhaps because it appears incorrect. However, Drude’s prediction is shown to be correct for the conditions treated, demonstrating the importance of constraints in deriving equivalent circuits for distributed systems. The predicted nonreciprocity is not fundamental, but instead is an artifact of the misrepresentation of energy by an equivalent circuit. The application to modern equivalent circuits is discussed.Physicsbhm2113PhysicsArticlesEBEX: A Balloon-Borne Telescope for Measuring Cosmic Microwave Background Polarization
http://academiccommons.columbia.edu/catalog/ac:182983
Chapman, Danielhttp://dx.doi.org/10.7916/D87D2SZ5Fri, 06 Feb 2015 00:00:00 +0000EBEX is a long-duration balloon-borne (LDB) telescope designed to probe polarization signals in the cosmic microwave background (CMB). It is designed to measure or place an upper limit on the inflationary B-mode signal, a signal predicted by inflationary theories to be imprinted on the CMB by gravitational waves, to detect the effects of gravitational lensing on the polarization of the CMB, and to characterize polarized Galactic foreground emission. The payload consists of a pointed gondola that houses the optics, polarimetry, detectors and detector readout systems, as well as the pointing sensors, control motors, telemetry sytems, and data acquisition and flight control computers. Polarimetry is achieved with a rotating half-wave plate and wire grid polarizer. The detectors are sensitive to frequency bands centered on 150, 250, and 410 GHz. EBEX was flown in 2009 from New Mexico as a full system test, and then flown again in December 2012 / January 2013 over Antarctica in a long-duration flight to collect scientific data. In the instrumentation part of this thesis we discuss the pointing sensors and attitude determination algorithms. We also describe the real-time map making software, "QuickLook", that was custom-designed for EBEX. We devote special attention to the design and construction of the primary pointing sensors, the star cameras, and their custom-designed flight software package, "STARS" (the Star Tracking Attitude Reconstruction Software). In the analysis part of this thesis we describe the current status of the post-flight analysis procedure. We discuss the data structures used in analysis and the pipeline stages related to attitude determination and map making. We also discuss a custom-designed software framework called "LEAP" (the LDB EBEX Analysis Pipeline) that supports most of the analysis pipeline stages.Physics, Astrophysicsdc2441PhysicsDissertationsSlave Mode Expansion for Obtaining Ab Initio Interatomic Potentials and its Applications
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Ai, Xinyuanhttp://dx.doi.org/10.7916/D85D8QNZFri, 06 Feb 2015 00:00:00 +0000Having an interatomic potential overcomes limitations within DFT since it has a negligible cost in computing material properties while DFT is severely restricted by its computational cost when carrying out such tasks. In this thesis, we propose a new approach for creating an interatomic potential based on the Taylor series expansion of the crystal energy as a function of its nuclear displacements. We enlarge the dimensionality of the existing displacement space and form new variables (ie. slave modes) which transform like irreducible representations of the point group and satisfy homogeneity of free space. Standard group theoretical techniques can then be applied to deduce the non-zero expansion coefficients a priori. At a given order, the translation group can be used to contract the products and eliminate terms which are not linearly independent, resulting in a final set of slave mode products. By the end of the day, one ends up with an expansion that satisfies lattice symmetry and its number of coefficients is much smaller than that of a common Taylor series expansion. While the expansion coefficients can be computed in a variety of ways, we demonstrate that finite difference is effective up to fifth order. On the other hand, we demonstrate the power of the method in the strongly anharmonic systems PbTe and graphene. All anharmonic terms within an octahedron are computed up to fourth order for PbTe, while those within a hexagon are computed up to fourth order and dimer terms are computed at fifth order for graphene. In addition, for PbTe, a proper unitary transformation of its potential demonstrates that the vast majority of the anharmonicity can be attributed to just two terms, indicating that a minimal model of phonon interactions is achievable. The ability to straightforwardly generate polynomial potentials will allow precise simulations at length and time scales which were previously unrealizable.Physicsxa2108PhysicsDissertationsSearch for Heavy Down-Type Vector-Like Quarks in the Lepton-Plus-Jets Final State in 8 TeV proton-proton Collisions Using the ATLAS Detector at the LHC
http://academiccommons.columbia.edu/catalog/ac:182970
Hu, Diedihttp://dx.doi.org/10.7916/D8HX1BH7Thu, 05 Feb 2015 00:00:00 +0000This dissertation presents a search for pair production of heavy down-type vector-like quarks (VLQ B) using the full 2012 data set of proton-proton collisions at the center of mass 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 1/fb. Events with a single charged lepton, which can be either an electron or muon, missing transverse energy, at least six jets, at least one tagged as a b-jet, and at least one reconstructed hadronically-decaying W/Z boson candidate are analyzed. No significant deviation from the Standard Model background prediction is observed. Under the assumption that only the B->Wt/Zb/Hb decay modes are allowed, 95% CL upper limits are derived in the two dimensional plane of the B->Wt branching ratio versus B->Hb branching ratio. In the specific case where the branching ratios are consistent with a SU(2) singlet scenario, the observed (expected) 95% CL lower limit on the VLQ B mass is 640 GeV (505 GeV). In the chiral B scenario with a 100% branching ratio of the decay B->Wt, the observed (expected) 95% CL lower limit on the B mass is 814 GeV (756 GeV).Physicsdh2455PhysicsDissertationsTheoretical study of charge density waves in transition metal materials
http://academiccommons.columbia.edu/catalog/ac:178222
Okamoto, Junichihttp://dx.doi.org/10.7916/D8N0155MWed, 08 Oct 2014 00:00:00 +0000In this thesis we theoretically study new aspects of charge density waves in transition metal materials recently revealed by scanning tunneling microscopy measurements. The two important problems that we have investigated are the effects of orbital degeneracy on the formation of the charge-density waves in cobalt nanowires, and the effects of dilute but strongly pinning impurities on the charge-density wave in niobium diselenide. We first present an overview on charge-density waves, and then introduce a general theoretical model describing charge-density waves. We also explain several known results about disorder effects on charge-density waves. We briefly touch on the principle of scanning tunneling microscopy and its advantages compared to other experimental tools. Second, we discuss the physics of one-dimensional cobalt nanowires along with experimental results. We propose a theoretical model that is relevant to cobalt nanowires, and then analyze the model by two theoretical tools: mean-field theory and bosonization. Our results show that the multi-orbitals allow a spin-triplet interaction among electrons leading to different phase diagrams from the ones considered previously for similar models. Numerical results obtained by first-principles calculations are also briefly explained. Third, we consider the effects of dilute strong impurities on the charge-density wave in niobium diselenide, a transition metal dichalcogenide. We first explain the material and properties of its charge-density wave phase. Then, detailed analysis of a scanning tunneling microscopy measurement is presented. Next, we analytically and numerically study a phenomenological model relevant to the experiment. We show that the dilute strong impurities have little effects at large length scales compared to the average inter-impurity distance, leading to a topologically ordered phase with a (quasi-)long-range autocorrelation; this result is quite different from conventional pictures predicting short-range order with the proliferation of topological defects.Condensed matter physicsjo2267PhysicsDissertationsHolographic Jet Quenching
http://academiccommons.columbia.edu/catalog/ac:178237
Ficnar, Andrejhttp://dx.doi.org/10.7916/D8M04417Wed, 08 Oct 2014 00:00:00 +0000In this dissertation we study the phenomenon of jet quenching in quark-gluon plasma using the AdS/CFT correspondence. We start with a weakly coupled, perturbative QCD approach to energy loss, and present a Monte Carlo code for computation of the DGLV radiative energy loss of quarks and gluons at an arbitrary order in opacity. We use the code to compute the radiated gluon distribution up to n=9 order in opacity, and compare it to the thin plasma (n=1) and the multiple soft scattering (n=\infty) approximations. We furthermore show that the gluon distribution at finite opacity depends in detail on the screening mass and the mean free path. In the next part, we turn to the studies of how heavy quarks, represented as "trailing strings" in AdS/CFT, lose energy in a strongly coupled plasma. We study how the heavy quark energy loss gets modified in a "bottom-up" non-conformal holographic model, constructed to reproduce some properties of QCD at finite temperature and constrained by fitting the lattice gauge theory results. The energy loss of heavy quarks is found to be strongly sensitive to the medium properties. We use this model to compute the nuclear modification factor R_AA of charm and bottom quarks in an expanding plasma with Glauber initial conditions, and comment on the range of validity of the model. The central part of this thesis is the energy loss of light quarks in a strongly coupled plasma. Using the standard model of "falling strings", we present an analytic derivation of the stopping distance of light quarks, previously available only through numerical simulations, and also apply it to the case of Gauss-Bonnet higher derivative gravity. We then present a general formula for computing the instantaneous energy loss in non-stationary string configurations. Application of this formula to the case of falling strings reveals interesting phenomenology, including a modified Bragg-like peak at late times and an approximately linear path dependence. Based on these results, we develop a phenomenological model of light quark energy loss and use it compute the nuclear modification factor R_AA of light quarks in an expanding plasma. Comparison with the LHC pion suppression data shows that, although R_AA has the right qualitative structure, the overall magnitude is too low, indicating that the predicted jet quenching is too strong. In the last part of the thesis we consider a novel idea of introducing finite momentum at endpoints of classical (bosonic and supersymmetric) strings, and the phenomenological consequences of this proposal on the energy loss of light quarks. We show that in a general curved background, finite momentum endpoints must propagate along null geodesics and that the distance they travel in an AdS5-Schwarzschild background is greater than in the previous treatments of falling strings. We also argue that this leads to a more realistic description of energetic quarks, allowing for an unambiguous way of distinguishing between the energy in the dual hard probe and the energy in the color fields surrounding it. This proposal also naturally allows for a clear and simple definition of the instantaneous energy loss. Using this definition and the "shooting string" initial conditions, we develope a new formula for light quark energy loss. Finally, we apply this formula to compute the nuclear modification factor R_AA of light hadrons at RHIC and LHC, which, after the inclusion of the Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry, shows a reasonably good agreement with the recent data.Physics, Theoretical physics, Nuclear physicsaf2440PhysicsDissertationsProbing the response of 2D crystals by optical spectroscopy
http://academiccommons.columbia.edu/catalog/ac:178240
Li, Yileihttp://dx.doi.org/10.7916/D8319TGXWed, 08 Oct 2014 00:00:00 +0000Atomically thin two-dimensional crystals form a distinct and growing class of new materials. The electromagnetic response of a two-dimensional crystal provides direct access to its electronic properties. This thesis presents a series of experimental studies of the electromagnetic response of model two-dimensional crystals as probed by optical spectroscopy. Our aim is to obtain understanding of their intrinsic linear and nonlinear response and the many-body interactions in these materials, as well as to explore the potential to use the two-dimensional materials for sensing applications. In the two studies of graphene, we either removed contaminations from the environment to reveal the intrinsic response or intentionally applied adsorbates to investigate how the electrons interact with the extrinsic molecules. In the first study, we obtained ultra-clean graphene using hexagonal boron nitride as the substrate, which allowed us to probe using Raman spectroscopy the intrinsic electron-phonon and electron-electron interactions free from substrate induced sample inhomogeneity. In a second study, we demonstrate a strong near-field electromagnetic interaction of graphene plasmons with the vibrations of adsorbed molecules. Our results reveal the potential of graphene for molecular sensing. In our investigations of the monolayer transition metal dichalcogenides, we performed measurements of the linear and the second-order nonlinear dielectric response. From the linear dielectric response, we demonstrate strong light-matter interactions even for a single layer of these materials. Several trends in the excitonic properties of this group of materials were obtained from the measured dielectric function. In the nonlinear optical study, we observed a large enhancement of the second-harmonic signal from monolayers as compared to the bulk sample, a consequence of the breaking of the inversion symmetry present in the bulk. In addition to the results for monolayers, we describe the behavior of few-layer materials, where the symmetry properties change layer by layer. For monolayers (and samples of odd layer thickness with broken inversion symmetry), the strong and anisotropic second-harmonic response provides a simple optical probe of crystallographic orientation. In the magneto-optic study of transition metal dichalcogenide monolayers, we demonstrate the induction of valley splitting and polarization by the application of an external magnetic field. The interaction of the valleys with the magnetic field reflects their non-zero magnetic moments, which are compared to theoretical models. We further clarify the electronic configuration of the charged excitons and important many-body corrections to the trion binding energy through the control of valley polarization achieved by the external magnetic field.Condensed matter physics, Opticsyl2673Physics, Electrical EngineeringDissertationsTowards inducing superconductivity into graphene
http://academiccommons.columbia.edu/catalog/ac:178213
Efetov, Dmitri K.http://dx.doi.org/10.7916/D8VX0F3TTue, 07 Oct 2014 00:00:00 +0000Graphenes transport properties have been extensively studied in the 10 years since its discovery in 2004, with ground-breaking experimental observations such as Klein tunneling, fractional quantum Hall effect and Hofstadters butterfly. Though, so far, it turned out to be rather poor on complex correlated electronic ground states and phase transitions, despite various theoretical predictions. The purpose of this thesis is to help understanding the underlying theoretical and experimental reasons for the lack of strong electronic interactions in graphene, and, employing graphenes high tunability and versatility, to identify and alter experimental parameters that could help to induce stronger correlations. In particular graphene holds one last, not yet experimentally discovered prediction, namely exhibiting intrinsic superconductivity. With its vanishingly small Fermi surface at the Dirac point, graphene is a semi-metal with very weak electronic interactions. Though, if it is doped into the metallic regime, where the size of the Fermi surface becomes comparable to the size of the Brillouin zone, the density of states becomes sizeable and electronic interactions are predicted to be dramatically enhanced, resulting in competing correlated ground states such as superconductivity, magnetism and charge density wave formation. Following these predictions, this thesis first describes the creation of metallic graphene at high carrier doping via electrostatic doping techniques based on electrolytic gates. Due to graphenes surface only properties, we are able to induce carrier densities above n>10¹⁴cm⁻²(εF>1eV) into the chemically inert graphene. While at these record high carrier densities we yet do not observe superconductivity, we do observe fundamentally altered transport properties as compared to semi-metallic graphene. Here, detailed measurements of the low temperature resistivity reveal that the electron-phonon interactions are governed by a reduced, density dependent effective Debey temperature - the so-called Bloch-Grüneisen temperature ΘBG. We also probe the transport properties of the high energy sub-bands in bilayer graphene by electrolyte gating. Furthermore we demonstrate that electrolyte gates can be used to drive intercalation reactions in graphite and present an all optical study of the reaction kinetics during the creation of the graphene derived graphite intercalation compound LiC₆, and show the general applicability of the electrolyte gates to other 2-dimensional materials such as thin films of complex oxides, where we demonstrate gating dependent conductance changes in the spin-orbit Mott insulator Sr₂IrO₄. Another, entirely different approach to induce superconducting correlations into graphene is by bringing it into proximity to a superconductor. Although not intrinsic to graphene, Cooper pairs can leak in from the superconductor and exist in graphene in the form of phase-coherent electron-hole states, the so-called Andreev states. Here we demonstrate a new way of fabricating highly transparent graphene/superconductor junctions by vertical stacking of graphene and the type-II van der Waals superconductor NbSe₂. Due to NbSe₂'s high upper critical field of Hc₂= 4 T we are able to test a long proposed and yet not well understood regime, where proximity effect and quantum Hall effect coexist.Physics, Nanosciencede2175PhysicsDissertationsElectronic and optical properties of titanate-based oxide heterostructures
http://academiccommons.columbia.edu/catalog/ac:178191
Park, Se Younghttp://dx.doi.org/10.7916/D8959G33Tue, 30 Sep 2014 00:00:00 +0000In this thesis we study properties of transition metal oxide heterostructures and superlattices, including electronic structures, optical responses, and metal-insulator transitions. We start with a general discussion of the properties of transition metal oxides, primarily ABO₃ (A: rare earth ion, B: transition metal, O:oxygen) perovskites. We introduce the effect of A-site substitution on the electronic and magnetic properties in bulk perovskites, followed by the basic properties of oxide heterostructures and superlattices composed of two different ABO₃ perovskites focusing on the metal insulator transitions and properties of the interface electron gas. Next, we present calculations of the charge density profile, subband occupancy and ellipsometry spectra of the electron gas at the LaAlO₃/SrTiO₃ interface. The calculations employ self-consistent Hartree and random phase approximations. We discuss the dependence of spatial structure and subband occupancy on the magnitude of the polarization charge at the interface and spatial structure of the dielectric constant. The response to applied AC electric fields is calculated and the results are presented in terms of the ellipsometry angles. Our results show a dip in the ellipsometry spectrum near the longitudinal optic phonon frequency of the SrTiO₃ and a peak at higher energy, which are related to the in-plane Drude response and the out-of-plane plasmon excitation, respectively. The relation of the features to the subband occupancies and the in-plane conductivities is given. We conclude with the study of thickness dependent metal-insulator transitions in superlattices composed of Mott insulating GdTiO₃ and band insulating SrTiO₃ using a first-principles GGA+U method. The structural and metal-insulator phase diagrams with respect to the number of unit cells, n, of SrTiO₃ and on-site correlation U are presented, showing that there are two different insulating phases for n>1 and n=1. For superlattices with n>1 the insulating phase involves both charge and orbital ordering with associated structures in Ti-O bond lengths but for n=1 superlattices, we find an insulating phase driven by orbital ordering within the quasi one-dimensional bonding bands across the SrO layer. The inconsistencies with experiment suggests the importance of the many-body correlations.Physicssp2829PhysicsDissertationsScanning Tunneling Microscopy Studies of Charge Density Waves in NbSe₂ and muSR studies of Nickel doping in BaFe₂As₂
http://academiccommons.columbia.edu/catalog/ac:189307
Arguello, Carlos Josehttp://dx.doi.org/10.7916/D8V69H48Tue, 30 Sep 2014 00:00:00 +0000Scanning Tunneling Microscopy is a very powerful technique to study electronic properties of condensed matter systems at the nanoscale. Part I of this thesis describes my work on Charge Density Waves (CDW) in NbSe₂. NbSe₂ is a layered dichalcogenide that has a CDW phase below 33K. We describe our study of the phase transition from the normal phase to the CDW phase at atomic scales. This is more relevant in light of recent discoveries of charge order in cuprates. Brand new research has shed some light about the relationship between the pseudogap phase, charge order and superconductivity in cuprates. The behavior of the CDW phase in NbSe₂ described in chapter 3 is strongly reminiscent of this physics of cuprates. NbSe₂ is an excellent test bed for the study of the effect of impurities in correlated phases. In chapter 4 we revisit the cause of CDW formation in NbSe₂. By including a very dilute concentration of impurities, we obtain information of the electronic bands of the material in the CDW phase. Based on this information, we are able to discuss the relationship between nesting, electron-phonon coupling and CDW in NbSe₂. We demonstrate that by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wavevector and energy dependence of the important electronic scattering processes. Part II focuses on Muon Spin Rotation and its application to the study of high-Tc superconductors. We describe our muSR studies on Nickel doped BaFe₂As₂. By analyzing several doping concentrations, we explore the phase diagram in the antiferromagnetic and in the superconducting phases. This discussion also includes a detailed discussion of a doping concentration which falls in-between the AF and the SC phase.Condensed matter physicscja2119PhysicsDissertationsK_L-K_S mass difference from lattice QCD
http://academiccommons.columbia.edu/catalog/ac:177236
Yu, Jiangleihttp://dx.doi.org/10.7916/D8F47MB1Tue, 12 Aug 2014 00:00:00 +0000The K_L-K_S mass difference is a promising quantity to reveal new phenomena which lie outside the standard model. A state-of-art perturbation theory calculation has be en performed at next-to-next-to-leading order (NNLO) and a 40% error is quoted in the final result. We develop and demonstrate non-perturbative techniques needed to calculate the K_L-K_S mass difference, ΔM_K, in lattice QCD and carry out exploratory calculations. The calculations are performed on a 2+1 flavor, domain wall fermion, 16³ x 32 ensemble with a 421 Mev pion and a 24³ x 64 lattice ensemble with a 329 MeV pion. In the $16^3$ lattice calculation, we drop the double penguin diagrams and the disconnected diagrams. The short distance part of the mass difference in a 2+1 flavor calculation contains a quadratic divergence cut off by the lattice spacing. Here, this quadratic divergence is eliminated through the Glashow-Iliopoulos-Maiani (GIM) mechanism by introducing a quenched charm quark. We obtain a mass difference ΔM_K which ranges from 6.58(30) x 10⁻¹² MeV to 11.89(81) x 10⁻¹² MeV for kaon masses varying from 563 MeV to 839 MeV. On the 24³ lattice, we include all the diagrams and perform a full calculation. Our result is for a case of unphysical kinematics with pion, kaon and charmed quark masses of 330, 575 and 949 MeV respectively. We obtain ΔM_K=3.19(41)(96) x 10⁻¹² MeV, quite similar to the experimental value. Here the first error is statistical and the second is an estimate of the systematic discretization error. An interesting aspect of this calculation is the importance of the disconnected diagrams, a dramatic failure of the OZI rule.Particle physicsjy2379PhysicsDissertationsA Search for tt Resonances in the Single Lepton Final State with the ATLAS Experiment
http://academiccommons.columbia.edu/catalog/ac:177112
Altheimer, Andrew Davidhttp://dx.doi.org/10.7916/D8XG9PBDWed, 06 Aug 2014 00:00:00 +0000A search for undiscovered particles decaying into top-antitop quark pairs produced in proton-proton collisions with the ATLAS experiment at the Large Hadron Collider utilizing 20.3 fb$^-1$ of data collected at $\sqrt{s}=8$ \tev\ center-of-mass energy during the 2012 data taking period is presented. The invariant mass spectrum of events containing multiple jets, exactly one lepton, and missing transverse energy and which are consistent with the decay of a top-antitop quark pair is studied and found to be consistent with that predicted by the Standard Model. Upper limits on the production cross section times branching ratio of several benchmark signal models are set at a 95\% confidence level.Particle physicsada2129PhysicsDissertationsThe Chiral and U(1)_A Symmetries of the QCD Phase Transition using Chiral Lattice Fermions
http://academiccommons.columbia.edu/catalog/ac:175885
Lin, Zhongjiehttp://dx.doi.org/10.7916/D86D5R4TMon, 07 Jul 2014 00:00:00 +0000With regard to the nature of the finite-temperature QCD phase transition and the fate of the chiral and anomalous axial symmetries associated with it, we present in this thesis two parallel sets of investigations into the QCD phase transition region between 139 and 195 MeV. Both studies adopt the Iwasaki gauge action augmented with the dislocation suppression determinant ratio with 2+1 flavors of chiral fermions. This choice of lattice action accurately reproduces the SU(2)_L × SU(2)_R and U(1)_A symmtries of the continuum. The first study simulates QCD thermodynamics on a line of constant physics that represents 200 MeV pions and physical kaons using domain wall fermions (DWF) at three space-time volumes: 16³ × 8, 24³ × 8, and 32³ × 8, where the largest volume varies in linear size between 5.6 fm (at T = 139 MeV) and 4.0 fm (at T = 195 MeV). The chiral condensates, connected and disconnected susceptibilities and the Dirac eigenvalue spectrum are reported and compared between different volumes as well as with the staggered results. We find a pseudo-critical temperature, T_c , of approximately 165 MeV and strong finite volume dependence below T_c. Clear evidence is seen for U(1)_A symmetry breaking above T_c which is quantitatively explained by the measured density of near-zero modes in accordance with the dilute instanton gas approximation. The second study targets on a line of constant physics with pions of physical mass, which is the very first study using a chiral lattice fermion formulaation. We continue to use the basic setup from the m_π ≈ 200 MeV simulations, except that we use a generalized form of domain wall fermions, known as the M ̈bius fermions, to further reduce the residual chiral symmetry breaking present in the domain wall formulation with finite extent in the fifth dimension. Preliminary results including the chiral condensates and the susceptibilities are reported for two space-time volumes of 32³ × 8 and 64³ × 8. We observe a dramatic increase in the disconnected susceptibilities and a shift in the pseudo-critical temperature from 165 MeV to about 154 MeV, when the pion mass is decreased from 200 MeV to 135 MeV.PhysicsPhysicsDissertationsNegative Modes in Vacuum Decay
http://academiccommons.columbia.edu/catalog/ac:176867
Lee, Hak Joonhttp://dx.doi.org/10.7916/D84X55Z3Mon, 07 Jul 2014 00:00:00 +0000A vacuum, a classically stable state, can decay to another vacuum by virtue of quantum tunneling. Although vacuum decay is an interesting topic itself in field theory, when it combines with gravity, it has wider applications and plays an essential role to understand the very early universe. The semi-classical solution of vacuum decay is well-described by the WKB approximation both in flat space and in curved space. Meanwhile, if we consider the configuration space of fields, we encounter many kinds of problems related with gravitational fields since the mode spectrum in curved space turns out to have an infinite number of negative modes in a de Sitter background. Despite of infinitely many negative modes, the regime in the weak gravity limit can be smoothly connected to flat space. To understand this, I discuss the nature of vacuum decay and various topics about negative modes in this thesis.Theoretical physics, Particle physicshl2406PhysicsDissertationsDiscriminative topological features reveal biological network mechanisms
http://academiccommons.columbia.edu/catalog/ac:174771
Middendorf, Manuel; Ziv, Etay; Adams, Carter; Hom, Jennifer C.; Koytcheff, Robin; Levovitz, Chaya; Woods, Gregory; Chen, Linda; Wiggins, Chris H.http://dx.doi.org/10.7916/D8VD6WKKFri, 06 Jun 2014 00:00:00 +0000Background: Recent genomic and bioinformatic advances have motivated the development of numerous network models intending to describe graphs of biological, technological, and sociological origin. In most cases the success of a model has been evaluated by how well it reproduces a few key features of the real-world data, such as degree distributions, mean geodesic lengths, and clustering coefficients. Often pairs of models can reproduce these features with indistinguishable fidelity despite being generated by vastly different mechanisms. In such cases, these few target features are insufficient to distinguish which of the different models best describes real world networks of interest; moreover, it is not clear a priori that any of the presently-existing algorithms for network generation offers a predictive description of the networks inspiring them. Results: We present a method to assess systematically which of a set of proposed network generation algorithms gives the most accurate description of a given biological network. To derive discriminative classifiers, we construct a mapping from the set of all graphs to a high-dimensional (in principle infinite-dimensional) "word space". This map defines an input space for classification schemes which allow us to state unambiguously which models are most descriptive of a given network of interest. Our training sets include networks generated from 17 models either drawn from the literature or introduced in this work. We show that different duplication-mutation schemes best describe the E. coli genetic network, the S. cerevisiae protein interaction network, and the C. elegans neuronal network, out of a set of network models including a linear preferential attachment model and a small-world model. Conclusions: Our method is a first step towards systematizing network models and assessing their predictability, and we anticipate its usefulness for a number of communities.Bioinformaticsjch149, chw2Applied Physics and Applied Mathematics, Physics, MathematicsArticlesThe Physics of Ultracold S₂ Molecules: Optical Production and Precision Measurement
http://academiccommons.columbia.edu/catalog/ac:173497
Osborn, Christopherhttp://dx.doi.org/10.7916/D8GH9G16Fri, 25 Apr 2014 00:00:00 +0000Ultracold molecules provide an exciting testing ground for studies of fundamental interactions, new states of matter, and metrology. Diatomic molecules based on two-electron atoms are especially suitable for precise tests of interatomic interactions, molecular quantum electrodynamics, electron-proton mass ratio variations, and other measurements in molecular and fundamental physics. This thesis describes the construction of a new strontium apparatus, from initial vacuum system setup through characterization of ultracold atom samples, followed by a new method of efficient, all-optical production of ultracold ^88Sr₂ molecules in an optical lattice, with detection via optical fragmentation. High-Q spectra of the weakly bound molecules in magnetic fields are studied, yielding precise binding energies, anomalously large molecular g factors resulting from large nonadiabatic effects, and strongly enhanced magnetic susceptibility. The thesis then concludes with an outlook on future experiments in our lab, including studies of forbidden molecular transitions, and longer term studies of fundamental physics from deeply bound Sr₂.PhysicsPhysicsDissertationsGravitation and Multimessenger Astrophysics
http://academiccommons.columbia.edu/catalog/ac:175203
Bartos, Imrehttp://dx.doi.org/10.7916/D8FT8J3BWed, 16 Apr 2014 00:00:00 +0000Gravitational waves originate from the most violent cosmic events, which are often hidden from traditional means of observation. Starting with the first direct observation of gravitational waves in the coming years, astronomy will become richer with a new messenger that can help unravel many of the yet unanswered questions on various cosmic phenomena. The ongoing construction of advanced gravitational wave observatories requires disruptive innovations in many aspects of detector technology in order to achieve the sensitivity that lets us reach cosmic events. We present the development of a component of this technology, the Advanced LIGO Optical Timing Distribution System. This technology aids the detection of relativistic phenomena through ensuring that time, at least for the observatories, is absolute. Gravitational waves will be used to look into the depth of cosmic events and understand the engines behind the observed phenomena. As an example, we examine some of the plausible engines behind the creation of gamma ray bursts. We anticipate that, by reaching through shrouding blastwaves, efficiently discovering off-axis events, and observing the central engine at work, gravitational wave detectors will soon transform the study of gamma ray bursts. We discuss how the detection of gravitational waves could revolutionize our understanding of the progenitors of gamma ray bursts, as well as related phenomena such as the properties of neutron stars. One of the most intriguing directions in utilizing gravitational waves is their combination with other cosmic messengers such as photons or neutrinos. We discuss the strategies and ongoing efforts in this direction. Further, we present the first observational constraints on joint sources of gravitational waves and high energy neutrinos, the latter of which is created in relativistic plasma outflows, e.g., in gamma ray burst progenitors. High energy neutrinos may be created inside a relativistic outflow burrowing its way out of a massive star from the star's collapsed core. We demonstrate how the detection of high energy neutrinos can be used to extract important information about the supernova/gamma-ray burst progenitor structure. We show that, under favorable conditions, even a few neutrinos are sufficient to probe the progenitor structure, opening up new possibilities for the first detections, as well for progenitor population studies. We present the science reach and method of an ongoing search for common sources of gravitational waves and high energy neutrinos using the initial LIGO/Virgo detectors and the partially completed IceCube detector. We also present results on the sensitivity of the search. We argue that such searches will open the window onto source populations whose electromagnetic emission is hardly detectable.Physics, Astrophysicsib2179PhysicsDissertationsA Search For Electron Antineutrino Disappearance with the Double Chooz Far Detector
http://academiccommons.columbia.edu/catalog/ac:172267
Toups, Matthew Henryhttp://dx.doi.org/10.7916/D8MP51B9Tue, 01 Apr 2014 00:00:00 +0000We present a search for electron antineutrino disappearance at the Chooz nuclear power plant in Chooz, France. Using the Double Chooz far detector and 101.5 days of detector run time, we measure sin^2(2θ(subscript 13)) = 0.086 ± 0.041 (stat.) ± 0.030 (syst.) from a rate and shape fit. A combined analysis of T2K and Double Chooz data finds that sin^2(2θ(subscript 13)) = 0 is excluded at the 3σ level.Particle physics, Physicsmht2114PhysicsDissertationsThe E and B EXperiment: A balloon-borne cosmic microwave background anisotropy probe
http://academiccommons.columbia.edu/catalog/ac:171485
Hillbrand, Seth Nathanielhttp://dx.doi.org/10.7916/D8KD1VZQFri, 07 Mar 2014 00:00:00 +0000The E and B Experiment (EBEX), is a balloon-borne sub-orbital cosmic microwave background polarimeter, designed to measure polarization levels in the microwave spectrum. EBEX recently completed an 11-day Antarctic long duration balloon (LDB) science flight in January, 2013. ~1000 transition edge sensor bolometric detectors in three frequency bands centered at 150, 250 and 410 GHz sampled a large segment of the southern sky. Over 1.5TB of data were collected during the LDB flight. In this thesis, we describe the design and performance of the EBEX software components monitoring and controlling the system during the flight, including automation, telemetry, data storage and readout array management. We also describe the design and development of a novel attitude reconstruction system for a balloon-borne pointed observation platform based on a daytime star camera and 3-axis gyroscopes. The data gathered during the LDB flight are analyzed and the results presented showing attitude reconstruction error at less than 20" RMS for an 80 second interval.Physicssnh2103PhysicsDissertationsSearch for Non-Pointing Photons in the Diphoton and Missing Transverse Energy Final State in 7 TeV pp Collisions Using the ATLAS Detector
http://academiccommons.columbia.edu/catalog/ac:171495
Nikiforou, Nikiforoshttp://dx.doi.org/10.7916/D8668B78Fri, 07 Mar 2014 00:00:00 +0000A search for photons originating in the decay of a neutral long-lived particle produced in proton-proton collisions at sqrt(s) = 7 TeV is presented. The search was performed in the diphoton plus missing transverse energy final state, using the full data sample of 4.8 fb-1 of 7 TeV proton-proton collisions collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The analysis exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction of photons, and utilizes the excellent time resolution of the calorimeter as an independent cross-check of the results. The search was conducted in the context of Gauge Mediated Supersymmetry Breaking models, where the lightest neutralino is the next-to-lightest supersymmetric particle and has a finite lifetime. In the family of models investigated, supersymmetric particles are produced in pairs due to R-parity conservation, eventually decaying to a pair of neutralinos, each subsequently decaying to a photon and a gravitino. The gravitinos escape the detector, giving rise to missing energy, while the photons can appear not to originate from the primary vertex of the event, and are measured with a delay with respect to the collision time. No excess was observed above the background expected from Standard Model processes. The results were used to set exclusion limits at 95% CL in the two-dimensional parameter space defined by the supersymmetry breaking scale and the lifetime of the lightest neutralino.Particle physicsnn2221PhysicsDissertationsThe Physics of Ultracold Sr_2 Molecules: Optical Production and Precision Measurement
http://academiccommons.columbia.edu/catalog/ac:173488
Osborn, Christopherhttp://dx.doi.org/10.7916/D8FT8J2WTue, 04 Mar 2014 00:00:00 +0000Ultracold molecules provide an exciting testing ground for studies of fundamental interactions, new states of matter, and metrology. Diatomic molecules based on two-electron atoms are especially suitable for precise tests of interatomic interactions, molecular quantum electrodynamics, electron-proton mass ratio variations, and other measurements in molecular and fundamental physics. This thesis describes the construction of a new strontium apparatus, from initial vacuum system setup through characterization of ultracold atom samples, followed by a new method of efficient, all-optical production of ultracold ^{88}Sr_2 molecules in an optical lattice, with detection via optical fragmentation. High-Q spectra of the weakly bound molecules in magnetic fields are studied, yielding precise binding energies, anomalously large molecular g factors resulting from large nonadiabatic effects, and strongly enhanced magnetic susceptibility. The thesis then concludes with an outlook on future experiments in our lab, including studies of forbidden molecular transitions, and longer term studies of fundamental physics from deeply bound Sr_2.PhysicsPhysicsDissertationsSearch for the Standard Model Higgs boson in Z + γ; final states with the ATLAS detector at the LHC
http://academiccommons.columbia.edu/catalog/ac:169799
Tian, Fenghttp://dx.doi.org/10.7916/D8G73BP1Mon, 03 Feb 2014 00:00:00 +0000This dissertation describes a search for the Standard Model Higgs boson in Z+photon channel with the ATLAS detector at the LHC using 4.6 fb⁻¹ of proton-proton collisions at center of mass energy 7 TeV in 2011 and 20.7 fb⁻¹ of proton-proton collisions at center of mass energy 8 TeV in 2012. The distribution of the mass difference between M_Zphoton and M_Z is compared to the Standard Model (SM) background expectations. No significant deviation from the SM prediction is observed and the upper limits on the signal strength μ of a Higgs boson with a mass between 120 and 150 GeV are derived. The expected exclusion limits at 95% confidence level range between 7.3 and 22.3 times the predicted Standard Model cross section. The observed exclusion limits range between 5.3 and 38.7 times the Standard Model cross section. For a Higgs boson with mass of 125.5 GeV, the expected and observed limits are 13.2 and 17.0 times the Standard Model respectively.Particle physicsft2181PhysicsDissertationsInclusive jet production in ultrarelativistic proton-nucleus collisions
http://academiccommons.columbia.edu/catalog/ac:182024
Perepelitsa, Dennishttp://dx.doi.org/10.7916/D82V2D2WWed, 22 Jan 2014 00:00:00 +0000High-pT processes in proton- and deuteron-nucleus collisions at TeV energies are the best presently available way to study the partonic structure of the nucleus in a high-density regime. Jet production over a wide range of phase space can significantly constrain the current knowledge of nuclear parton distribution functions (nPDFs), which are substantially less well understood than the corresponding PDFs in protons and which have only recently begun to be treated in a spatially-dependent way. An accurate knowledge of nPDFs is crucial for a definitive control of perturbative processes in a cold nuclear environment, since high-pT probes are used to quantitatively investigate the hot QCD matter created in ultrarelativistic nucleus-nucleus collisions. Furthermore, jets from low Bjorken-x partons can probe the transition from the dilute to saturated nuclear regimes.Physics, Nuclear physics, Particle physicsdvp2102PhysicsDissertationsObservational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS
http://academiccommons.columbia.edu/catalog/ac:166933
Senturk, Gunes Demethttp://hdl.handle.net/10022/AC:P:22118Mon, 04 Nov 2013 00:00:00 +0000Blazars are active galactic nuclei with a relativistic jet directed towards the observer's line of sight. Characterization of the non-thermal continuum emission originating from the blazar jet is currently an essential question in high-energy astrophysics. A blazar spectral energy distribution (SED) has a typical double-peaked shape in the flux vs. energy representation. The low-energy component of the SED is well-studied and thought to be due to synchrotron emission from relativistic electrons. The high-energy component, on the other hand, is still not completely understood and the emission in this part of the blazar spectrum can extend to energies as high as tera electron volts in some objects. This portion of the electromagnetic spectrum is referred to as the very-high-energy (VHE or TeV, E > 0.1 TeV) regime. At the time of this writing, more than half a hundred blazars have been detected to emit TeV gamma rays, representing the high energy extreme of these objects and constituting a population of its own. Most of these TeV blazars have also been detected in the high-energy (HE or GeV, 0.1 GeV < E < 0.1 TeV) gamma-ray range. In this work, we report on our discovery of the TeV emission from the blazar RBS 0413 and perform a detailed data analysis on this source, including contemporaneous multi-wavelength observations to characterize the broad-band SED and test various emission models for the high-energy component. Further, we extend our focus on the high-energy component to all archival TeV-detected blazars and study their spectral properties in the framework of GeV and TeV gamma-ray observations. To do this, we assemble for the first time the GeV and TeV spectra of a complete sample of TeV-detected blazars available in the archive to date. In the Appendix we present an analysis method for improved observations of large zenith angle targets with VERITAS.Physics, Astrophysicsgds2110PhysicsDissertationsThe Study of Transition Metal Oxides using Dynamical Mean Field Theory
http://academiccommons.columbia.edu/catalog/ac:166930
Dang, Hung Thehttp://hdl.handle.net/10022/AC:P:22116Mon, 04 Nov 2013 00:00:00 +0000In this thesis, we study strong electron correlation in transition metal oxides. In the systems with large Coulomb interaction, especially the on-site interaction, electrons are much more correlated and cannot be described using traditional one-electron picture, thus the name "strongly correlated systems". With strong correlation, there exists a variety of interesting phenomena in these systems that attract long-standing interests from both theorists and experimentalists. Transition metal oxides (TMOs) play a central role in strongly correlated systems, exhibiting many exotic phenomena. The fabrication of heterostructures of transition metal oxides opens many possible directions to control bulk properties of TMOs as well as revealing physical phases not observed in bulk systems. Dynamical mean-field theory (DMFT) emerges as a successful numerical method to treat the strong correlation. The combination of density functional and dynamical mean-field theory (DFT+DMFT) is a prospective ab initio approach for studying realistic strongly correlated materials. We use DMFT as well as DFT+DMFT methods as main approaches to study the strong correlation in these materials. We focus on some aspects and properties of TMOs in this work. We study the magnetic properties in bulk TMOs and the possibility of enhancing the magnetic order in TMO heterostructures. We work on the metallic/insulating behaviors of these systems to understand how the metal-insulator transition depends on the intrinsic parameters of materials. We also investigate the effect of a charged impurity to the neighborhood of a correlated material, which can be applied, for example, to the study of muon spin relaxation measurements in high-Tc superconductors.PhysicsPhysicsDissertationsAn atom trap trace analysis (ATTA) system for measuring ultra-low contamination by krypton in xenon dark matter detectors
http://academiccommons.columbia.edu/catalog/ac:166773
Yoon, Taehyunhttp://hdl.handle.net/10022/AC:P:22057Thu, 31 Oct 2013 00:00:00 +0000The XENON dark matter experiment aims to detect hypothetical weakly interacting massive particles (WIMPs) scattering off nuclei within its liquid xenon (LXe) target. The trace 85Kr in the xenon target undergoes beta-decay with a 687 keV end point and 10.8 year halflife, which contributes background events and limits the sensitivity of the experiment. In order to achieve the desired sensitivity, the contamination by krypton is reduced to the part per trillion (ppt) level by cryogenic distillation. The conventional methods are not well suited for measuring the krypton contamination at such a low level. In this work, we have developed an atom trap trace analysis (ATTA) device to detect the ultra-low krypton concentration in the xenon target. This project was proposed to the National Science Foundation (NSF) as a Major Research Instrumentation (MRI) development [Aprile and Zelevinsky, 2009] and is funded by NSF and Columbia University. The ATTA method, originally developed at Argonne National Laboratory, uses standard laser cooling and trapping techniques, and counts single trapped atoms. Since the isotopic abundance of 85Kr in nature is 1.5 × 10^-11, the 85Kr/Xe level is expected to be ~10^-23, which is beyond the capability of our method. Thus we detect the most abundant (57%) isotope 84Kr, and infer the 85Kr contamination from their known abundances. To avoid contamination by krypton, the setup is tested and optimized with 40Ar which has a similar cooling wavelength to 84Kr. Two main challenges in this experiment are to obtain a trapping efficiency high enough to detect krypton impurities at the ppt level, and to achieve the resolution to discriminate single atoms. The device is specially designed and adjusted to meet these challenges. After achieving these criteria with argon gas, we precisely characterize the efficiency of the system using Kr-Xe mixtures with known ratios, and find that ~90 minutes are required to trap one 84Kr atom at the 1-ppt Kr/Xe contamination. This thesis describes the design, construction, and experimental results of the ATTA project at Columbia University.Atomic physics, Astrophysicsty2182PhysicsDissertationsChemical Vapor Deposition Grown Pristine and Chemically Doped Monolayer Graphene
http://academiccommons.columbia.edu/catalog/ac:177571
Zhao, Liuyanhttp://hdl.handle.net/10022/AC:P:21666Wed, 18 Sep 2013 00:00:00 +0000Chemical vapor deposition growth has been a popular technique to produce large-area, high-quality monolayer graphene on Cu substrates ever since its first demonstration in 2009. Pristine graphene grown in such a way owns the natures of zero charge carriers and zero band gap. As an analogy to semi-conductor studies, substitutional doping with foreign atoms is a powerful way to tailor the electronic properties of this host materials. Within such a context, this thesis focuses on growing and characterizing both pristine and chemically-doped CVD grown monolayer graphene films at microscopic scales. We first synthesized pristine graphene on Cu single crystals in ultra-high-vacuum and subsequently characterized their properties by scanning tunneling microscopy/spectroscopy (STM/S), to learn the effects of Cu substrate crystallinity on the quality of graphene growth and understand the interactions between graphene films and Cu substrates. In the subsequent chapters, we chemically doped graphene with nitrogen (N) and boron (B) atoms, and characterized their topographic and electronic structures via STM/S. We found that both N and B dopants substitionally dope graphene films, and contribute electron and hole carriers, respectively, into graphene at a rate of approximately 0.5 carrier/dopant. Apart from this, we have made comparisons between N- and B-doped graphene films in aspects of topographic features, dopant distribution and electronic perturbations. In the last part of this thesis, we used Raman spectroscopy mapping to investigate the N dopant distribution within and across structural grains. Future experiments are also brief discussed at the end of the thesis.Condensed matter physics, Physicslz2227PhysicsDissertationsTopics in vacuum decay
http://academiccommons.columbia.edu/catalog/ac:165209
Abad, Ali Masoumi Khalilhttp://hdl.handle.net/10022/AC:P:21643Mon, 16 Sep 2013 00:00:00 +0000If a theory has more than one classically stable vacuum, quantum tunneling and thermal jumps make the transition between the vacua possible. The transition happens through a first order phase transition started by nucleation of a bubble of the new vacuum. The outward pressure of the truer vacuum makes the bubble expand and consequently eat away more of the old phase. In the presence of gravity this phenomenon gets more complicated and meanwhile more interesting. It can potentially have important cosmological consequences. Some aspects of this decay are studied in this thesis. Solutions with different symmetry than the generically used O(4) symmetry are studied and their actions calculated. Vacuum decay in a spatial vector field is studied and novel features like kinky domain walls are presented. The question of stability of vacua in a landscape of potentials is studied and the possible instability in large dimension of fields is shown. Finally a compactification of the Einstein-Maxwell theory is studied which can be a good lab to understand the decay rates in compactification models of arbitrary dimensions.Physicsam2809PhysicsDissertationsThe Effective Field Theory Approach to Fluid Dynamics, Modified Gravity Theories, and Cosmology
http://academiccommons.columbia.edu/catalog/ac:165139
Wang, Junpuhttp://hdl.handle.net/10022/AC:P:21602Fri, 13 Sep 2013 00:00:00 +0000The effective field theory approach is powerful in understanding the low energy phenomena without invoking the UV degrees of freedom. We construct a low energy Lagrangian for ordinary fluid systems (in constrast to superfluid), pure from symmetry considerations and EFT principles. The dynamical fields are the Goldstone excitations, associated with spontaneously broken spacetime translations. It is organized as derivatively coupled theory involving multiple scalar fields. This formalism enables us to study fluid's quantum mechanical properties and dissipative effects. Cosmological models can be built by naturally coupling the fluid EFT to gravity. From the EFT point of view, GR is the unique low energy theory for the spin-2 graviton field and any infrared modification corresponds to adding new degrees of freedom. We focus on two popular classes of modified gravity models, --- the chameleon like theories and the Galileon theory, --- and perform a few reliability checks for their qualifications as modified gravity theories. Furthermore, guiled by the EFT spirit, we develop a cosmological model where primordial inflation is driven by a `solid', defined, in a similar manner as the EFT of fluid. The symmetry breaking pattern differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard EFT description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations, and exhibits various unusual features.Physicsjw2551PhysicsDissertationsThe Effective Field Theory Approach to Fluid Dynamics, Modified Gravity Theories, and Cosmology
http://academiccommons.columbia.edu/catalog/ac:164394
Wang, Junpuhttp://hdl.handle.net/10022/AC:P:21391Wed, 21 Aug 2013 00:00:00 +0000The effective field theory approach is powerful in understanding the low energy phenomena without invoking the UV degrees of freedom. We construct a low energy Lagrangian for ordinary fluid systems (in constrast to superfluid), pure from symmetry considerations and EFT principles. The dynamical fields are the Goldstone excitations, associated with spontaneously broken spacetime translations. It is organized as derivatively coupled theory involving multiple scalar fields. This formalism enables us to study fluid's quantum mechanical properties and dissipative effects. Cosmological models can be built by naturally coupling the fluid EFT to gravity. From the EFT point of view, GR is the unique low energy theory for the spin-2 graviton field and any infrared modification corresponds to adding new degrees of freedom. We focus on two popular classes of modified gravity models, --- the chameleon like theories and the Galileon theory, --- and perform a few reliability checks for their qualifications as modified gravity theories. Furthermore, guiled by the EFT spirit, we develop a cosmological model where primordial inflation is driven by a `solid', defined, in a similar manner as the EFT of fluid. The symmetry breaking pattern differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard EFT description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations, and exhibits various unusual features.Physicsjw2551PhysicsDissertationsA Measurement of the Jet Multiplicity in Di-lepton Final States of ttbar Events
http://academiccommons.columbia.edu/catalog/ac:164089
Urbaniec, Dustin Henryhttp://hdl.handle.net/10022/AC:P:21263Tue, 06 Aug 2013 00:00:00 +0000A measurement of the jet multiplicity in di-leptonically decaying ttbar events (i.e. ttbar → (ee, μμ, eμ) + missing transverse momentum + jets) is presented, using 4.66 fb -1 of data collected from √ s = 7 TeV pp collisions at the CERN Large Hadron Collider. In order to mitigate the effects of systematic uncertainties associated with jet energy measurements, a measurement of the ratio of the number of di-lepton ttbar events with N + 2 jets to Z → ℓ ℓ + N jet events is also determined. The results are unfolded for detector effects and compared to the particle-level predictions of several Monte Carlo generators, showing generally good agreement.Particle physicsdhu2101PhysicsDissertationsCosmology with Weak Lensing Peaks
http://academiccommons.columbia.edu/catalog/ac:163342
Yang, Xiuyuanhttp://hdl.handle.net/10022/AC:P:21104Tue, 16 Jul 2013 00:00:00 +0000Recent studies have shown that the number counts of peaks in weak lensing (WL) surveys contain significant cosmological information. Motivated by this finding, in the first part of the thesis, we address two questions: (i) what is the physical origin of WL peaks; and (ii) how much information do the peaks contain beyond the traditional cosmological WL observable (the power spectrum). To investigate the first question, we use a suite of ray-tracing N-body simulations, in which we identify individual dark matter halos. We study the halos' contribution to the peaks. We find that high peaks are typically dominated by a single massive halo, while low peaks are created by galaxy shape noise, but with an important contribution from a line-of-sight projection of typically 4-8 halos. For the second question, we first compare the cosmological peak count distributions to those in a Gaussian random field. We find significant differences, both in the peak-count distributions themselves, as well as in how the distributions depend on cosmology, demonstrating that the peaks contain non-Gaussian information. To explicitly quantify the information content of the peaks beyond the power spectrum, we use the Fisher matrix method to forecast errors in the three-dimensional parameters space (σ_8, w, Ω_m). We find that when we combine the peaks and the power spectrum, the marginalized errors are a factor of about two smaller than from power spectrum alone. In the second part of the thesis, we address a major theoretical systematic error: the presence of baryons -- not included in the N-body simulations -- can affect the WL statistics (both peaks and power spectrum), and the inferred cosmological parameters. We apply a simplified model, which mimics the cooling and condensation of baryons at the centers of dark matter halos. In particular, we manually steepen the density profile of each dark matter halo identified in the N-body simulations, and repeat the ray-tracing procedure create WL maps in mock "baryonic'' universes. We then compare the peak count distributions and power spectra in these baryonic models to those from the pure DM models. We find that there is a large increase in the number of high peaks, but low peaks -- which contain most of the cosmological information -- are robust to baryons. Similarly, we find that the high--l power spectrum is increased, but the change in the low--l power spectrum is relatively modest. We then utilize a Monte Carlo approach to compute the joint, and in general, biased constraints on σ_8, w, Ω_m when the baryonic model is fit by the pure DM models. We find that: (i) constraints obtained from low peaks are nearly unbiased; (ii) high peaks yield large biases, but in different directions in parameter space than the biases from the power spectrum. Our first finding suggests it may be advantageous to use low peaks for analysis until the baryonic processes are better understood. However, our second finding suggests the possibility of "self-calibration'': simultaneously fitting astrophysical "nuisance'' parameters (describing lensing halo profiles) with cosmological parameters.Astrophysicsxy2117PhysicsDissertationsQuantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster
http://academiccommons.columbia.edu/catalog/ac:162983
Bartos, Imre; Mendes, Cesar; Akay, Turgay; Marka, Szabolcs; Mann, Richard S.http://hdl.handle.net/10022/AC:P:20975Wed, 10 Jul 2013 00:00:00 +0000Coordinated walking in vertebrates and multi-legged invertebrates such as Drosophila melanogaster requires a complex neural network coupled to sensory feedback. An understanding of this network will benefit from systems such as Drosophila that have the ability to genetically manipulate neural activities. However, the fly's small size makes it challenging to analyze walking in this system. In order to overcome this limitation, we developed an optical method coupled with high-speed imaging that allows the tracking and quantification of gait parameters in freely walking flies with high temporal and spatial resolution. Using this method, we present a comprehensive description of many locomotion parameters, such as gait, tarsal positioning, and intersegmental and left-right coordination for wild type fruit flies. Surprisingly, we find that inactivation of sensory neurons in the fly's legs, to block proprioceptive feedback, led to deficient step precision, but interleg coordination and the ability to execute a tripod gait were unaffected. - See more at: http://elife.elifesciences.org/content/2/e00231#sthash.6OwrygKl.dpufBiochemistry, Biophysics, Neurosciencesib2179, cm2729, ta2203, sm2375, rsm10Neurological Surgery, Physics, Biochemistry and Molecular BiophysicsArticlesPrecision Search for Muon Antineutrino Disappearance Oscillations Using a Dual Baseline Technique
http://academiccommons.columbia.edu/catalog/ac:162005
Cheng, Gary Chia Lihttp://hdl.handle.net/10022/AC:P:20634Fri, 07 Jun 2013 00:00:00 +0000A search for short baseline muon antineutrino disappearance with the SciBooNE and MiniBooNE experiments at Fermi National Accelerator Laboratory in Batavia, Illinois is presented. Short baseline muon antineutrino disappearance measurements help constrain sterile neutrino models. The two detectors observe muon antineutrinos from the same beam, therefore the combined analysis of their data sets serves to partially constrain some of the flux and cross section uncertainties. A likelihood ratio method was used to set a 90% confidence level upper limit on muon antineutrino disappearance that dramatically improves upon prior sterile neutrino oscillation limits in the Δm^2=0.1-100 eV^2 region.Physics, Particle physicsgcc2113PhysicsDissertationsControl study of two-particle correlations in heavy ion collisions at RHIC-PHENIX
http://academiccommons.columbia.edu/catalog/ac:161549
Vazquez, Erichttp://hdl.handle.net/10022/AC:P:20454Fri, 24 May 2013 00:00:00 +0000Measurements at the Relativistic Heavy Ion Collider (RHIC) have provided indirect measurements of jets in a heavy ion environment using the two- particle correlation method in the presence of a high-pT particle. These measurements have offered insight into the formation of a new state of dense nuclear matter called the Quark-Gluon Plasma (QGP) through the observation of jet quenching. However, the two-particle methodology has also shown to be biased towards di-jet production near the surface of the medium being created. Here, a detailed study using the PHENIX detector is provided, in an attempt to measure a more accurate jet-induced two-particle correlation measurement than previously published and to reduce the bias observed in two-particle correlation measurements. The reduction in surface bias emission is performed via the requirement of two antipodal high-pT particles (a.k.a. "2+1" correlation) in an attempt to control the production point of the di-jet. The measurements made in Au+Au collisions when compared to p+p collisions show that the method provides additional sensitivity to the jet quenching previously observed in two-particle correlation method.Nuclear physics, Physics, Particle physicsev2122PhysicsDissertationsThe Effective Field Theory Approach to Fluid Dynamics
http://academiccommons.columbia.edu/catalog/ac:161458
Endlich, Solomonhttp://hdl.handle.net/10022/AC:P:20419Thu, 23 May 2013 00:00:00 +0000In this thesis we initiate a systematic study of fluid dynamics using the effective field theory (EFT) program. We consider the canonical quantization of an ordinary fluid in an attempt to discover if there is some kind of quantum mechanical inconsistency with ordinary fluids at zero temperature. The system exhibits a number of peculiarities associated with the vortex degrees of freedom. We also study the dynamics of a nearly incompressible fluid via (classical) effective field theory. In the kinematical regime corresponding to near incompressibility (small fluid velocities and accelerations), compressional modes are, by definition, difficult to excite, and can be dealt with perturbatively. We systematically outline the corresponding perturbative expansion, which can be thought of as an expansion in the ratio of fluid velocity and speed of sound. This perturbation theory allows us to compute many interesting quantities associated with sound-flow interactions. Additionally, we also improve on the so-called vortex filament model, by providing a local field theory describing the dynamics of vortex-line systems and their interaction with sound, to all orders in perturbation theory. Next, we develop a cosmological model where primordial inflation is driven by a 'solid'. The low energy EFT describing such a system is just a less symmetric version of the action of a fluid---it lacks the volume preserving diffeomorphism. The symmetry breaking pattern of this system differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard effective field theory description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations. And finally, we introduce dissipative effects in the effective field theory of hydrodynamics. We do this in a model-independent fashion by coupling the long-distance degrees of freedom explicitly kept in the effective field theory to a generic sector that "lives in the fluid'', which corresponds physically to the microscopic constituents of the fluid. At linear order in perturbations, the symmetries, the derivative expansion, and the assumption that this microscopic sector is thermalized, allow us to characterize the leading dissipative effects at low frequencies via three parameters only, which correspond to bulk viscosity, shear viscosity, and---in the presence of a conserved charge---heat conduction. Using our methods we re-derive the Kubo relations for these transport coefficients.Theoretical physics, Physics, Condensed matter physicssge2104PhysicsDissertationsPrecision Lattice Calculation of Kaon Decays with Möbius Domain Wall Fermions
http://academiccommons.columbia.edu/catalog/ac:161136
Yin, Hantaohttp://hdl.handle.net/10022/AC:P:20325Tue, 14 May 2013 00:00:00 +0000We report our recent development in algorithms and progress in measurements in lattice QCD. The algorithmic development includes the forecasted force gradient integrator, and further theoretical development and implementation of the Möbius domain wall fermions. These new technologies make it practical to simulate large 48^3*96 and 64^3*128 lattice ensembles with (5.5fm)^3 boxes and 140MeV pion. The calculation was performed using the Möbius domain wall fermions and the Iwasaki gauge action. Simulated directly at physical quark masses, these ensembles are of great value for our ongoing and future lattice measurement projects. With the help of measurement techniques such as the eigCG algorithm and the all mode averaging method, we perform a direct, precise lattice calculation of the semileptonic kaon decay K→πlν using these newly generated high quality lattice ensembles. Our main result is the form factor f^+_{Kπ}(q^2) evaluated directly at zero momentum transfer q^2=0. Free of various systematic errors, this new result can be used to determine the CKM matrix element Vus to a very high precision when combined with experimental input. The calculation also provides results for various low energy strong interaction constants such as the pseudoscalar decay constants f_K and f_π, and the neutral kaon mixing matrix element B_K. These calculations are naturally performed by reusing the propagators calculated for the kaon semileptonic decay mentioned above. So they come with no or very low additional cost. The results allow us to also determine these important low energy constants on the lattice to unprecedented accuracy.Physics, Particle physics, Theoretical physicshy2242PhysicsDissertationsInferring Transcriptional and Post-Transcriptional Network Structure by Exploiting Natural Sequence Variation
http://academiccommons.columbia.edu/catalog/ac:188457
Fazlollahi, Minahttp://dx.doi.org/10.7916/D8XK8DXCTue, 14 May 2013 00:00:00 +0000Understanding how cellular processes of an organism translate its genome into its phenotype is one of the grand challenges in biology. Linkage studies seek to identify allelic variants that manifest themselves as phenotypic variation between individuals in a population. The advent of high-throughput genotyping and gene expression profiling technologies has made it possible to use messenger RNA levels as quantitative traits in linkage studies. This has created new opportunities to study genetic variation at the level of gene regulatory networks rather than individual genes. This thesis consists of four parts, each of which outlines a different strategy for integrating genome-wide expression data and genotype data in order to identify transcriptional and post-transcriptional regulatory mechanisms. The data for these analyses comes from segregating populations of Saccharomyces cerevisiae (baker’s yeast) as well as Caenorhabditis elegans (roundworm). The first study focused on inferring the in vitro binding specificity of RNA-binding proteins (RBPs). We first analyzed a recent compendium of in vivo mRNA binding data to model the sequence specificity of 45 yeast RBPs in the form of a position- specific affinity matrix (PSAM). We were able to recover known consensus nucleotide sequences for 12 RBPs and discovered novel binding preferences for 3 of the RBPs namely, Scp160p, Sik1p and Tdh3p. The second study aimed to identify transacting chromosomal loci that regulate expression of a large number of genes. Traditionally, such loci are discovered by first mapping expression quantitative loci (eQTLs) for individual genes, and then looking for so-called “eQTLs hotspots”. Our method avoids the first step by integrating information across all genes, leading to a more elegant method that has increased statistical power. For yeast, we recovered 70% of the reported eQTL hotspots from two independent studies, and discovered a new transacting locus on chromosome V. For worm, we detected six transacting loci, only two of which were previously reported as eQTL hotspots. The third study focused on post-transcriptional regulatory networks in yeast, by mapping the regulatory activity level of RNA binding proteins (RBPs) as a quantitative trait in so-called “aQTL” analysis. We used the collection of 15 sequence motifs with the associated mRNA region combinations that we obtained in our first study together with mRNA expression data to estimate RBP activities across yeast segregants. Consistent with a previous study, we recovered the MKT1 locus on chromosome XIV as a genetic modulator of Puf3p activity. We also discovered that Puf3p activity is modulated through distinct loci depending on whether it is binding to 50 or 30 untranslated region (UTR) of its target mRNAs. Furthermore, we identified a locus on chromosome XV that includes the IRA2 gene as a putative aQTL for Puf4p; this prediction was validated using expression data for an IRA2 allele replacement strain. Our fourth study focused on the detection of loci whose allelic variation modulates the in vivo regulatory connectivity between a transcription factor and its target genes. We call these loci connectivity QTLs or “cQTLs”. We mapped the DIG2 locus on chromosome IV as a cQTL for the transcription factor Ste12p. Dig2p is indeed a known inhibitor of yeast mating response activator Ste12p. The coding region of the DIG2 gene contains a single non-synonymous mutation (T83I). We are experimentally testing the functional impact of this mutation in allele replacement strains. We also identified the TAF13 locus as a putative modulator of GCN4p connectivity.Biomechanics, Physicsmf2235Physics, Biological SciencesDissertationsJet quenching in Quark Gluon Plasma: flavor tomography at RHIC and LHC by the CUJET model
http://academiccommons.columbia.edu/catalog/ac:160528
Buzzatti, Alessandrohttp://hdl.handle.net/10022/AC:P:20094Wed, 01 May 2013 00:00:00 +0000A new jet tomographic model and numerical code, CUJET, is developed in this thesis and applied to the phenomenological study of the Quark Gluon Plasma produced in Heavy Ion Collisions.Nuclear physicsab3091PhysicsDissertationsPrecision Lattice Calculation of Kaon Decays with Möbius Domain Wall Fermions
http://academiccommons.columbia.edu/catalog/ac:160818
Yin, Hantaohttp://hdl.handle.net/10022/AC:P:20151Wed, 01 May 2013 00:00:00 +0000We report our recent development in algorithms and progress in measurements in lattice QCD. The algorithmic development includes the forecasted force gradient integrator, and further theoretical development and implementation of the Möbius domain wall fermions. These new technologies make it practical to simulate large 48^3*96 and 64^3*128 lattice ensembles with (5.5fm)^3 boxes and 140MeV pion. The calculation was performed using the Möbius domain wall fermions and the Iwasaki gauge action. Simulated directly at physical quark masses, these ensembles are of great value for our ongoing and future lattice measurement projects.With the help of measurement techniques such as the eigCG algorithm and the all mode averaging method, we perform a direct, precise lattice calculation of the semileptonic kaon decay K→πlν using these newly generated high quality lattice ensembles. Our main result is the form factor f^+_{Kπ}(q^2) evaluated directly at zero momentum transfer q^2=0. Free of various systematic errors, this new result can be used to determine the CKM matrix element Vus to a very high precision when combined with experimental input. The calculation also provides results for various low energy strong interaction constants such as the pseudoscalar decay constants f_K and f_π, and the neutral kaon mixing matrix element B_K. These calculations are naturally performed by reusing the propagators calculated for the kaon semileptonic decay mentioned above. So they come with no or very low additional cost. The results allow us to also determine these important low energy constants on the lattice to unprecedented accuracy.Particle physics, Physics, Theoretical physicshy2242PhysicsDissertationsCharged Particle Multiplicity and Open Heavy Flavor Physics in Relativistic Heavy Ion Collisions at the LHC
http://academiccommons.columbia.edu/catalog/ac:159130
Chen, Yujiaohttp://hdl.handle.net/10022/AC:P:19740Fri, 12 Apr 2013 00:00:00 +0000In this thesis, two independent measurements are presented: the measure- ments of centrality dependence and pseudo-rapidity dependence of charged particle multiplicities, and the measurements of centrality dependence of open heavy flavor suppression. These measurements are carried out with the Pb+Pb collisions data at the LHC energy and = 2.76 TeV with the ATLAS detector. For the charged particle measurements, charged particles are reconstructed with two algorithms (2-point "tracklet" and full tracking) from the pixel detector only. Measurements are presented of the per-event charged particle density distribution, dNch/d η and the average charged par- ticle multiplicity in the pseudo-rapidity interval |η| <0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of partic- ipants is consistent with the lower √sNN results measured at RHIC. The shape of the dNch/η distribution is found to be independent of centrality within the systematic uncertainties of the measurement. For the open heavy flavor suppression measurements, muons identified by the muon spectrom- eter are classified as heavy flavor decays and background contributions by using a fitting procedure with templates from Monte Carlo samples. Results are presented for the per-event muon yield as a function of muon transverse momentum, pT, over the range of 4 pT 14 GeV. Over that momentum range single muon production results largely from heavy quark decays. The centrality dependence of the muon yields is characterized by the "central to peripheral" ratio, RCP. Using this measure, muon production from heavy quark decays is found to be suppressed by a centrality-dependent factor that increases smoothly from peripheral to central collisions. Muon production is suppressed by approximately a factor of two in central collisions relative to peripheral collisions. Within the experimental errors, the observed sup- pression is independent of muon pT for all centralities. Furthermore, the pT dependence of the relative muon yields in Pb+Pb collisions to p+p colli- sions with the same center of mass collision energy per nucleon is presented by the nuclear modification factor RAA, which is defined as the ratio of a spectrum from heavy ion collisions to the same but scaled spectrum from nucleon-nucleon collisions . The observed RAA has little dependence on pT within the uncertainties quoted here. The results for RAA indicate a factor of about 3 suppression in the yield of muons in the most central (0-10%) collisions compared to the p+p collisions.Physicsyc2420PhysicsDissertationsProbing static disorder in protein unfolding and chemical reactions by single-molecule force spectroscopy
http://academiccommons.columbia.edu/catalog/ac:161362
Kuo, Tzu-Linghttp://hdl.handle.net/10022/AC:P:19208Fri, 01 Mar 2013 00:00:00 +0000The work presented in this dissertation focuses on the kinetics of biomolecular reactions under mechanical force, including protein unfolding and disulfide-bond reduction, probed at the single-molecule level. The advent of single-molecule force spectroscopy has allowed the direct measure of force-dependent reaction rates, providing a powerful approach to extract the kinetic information and to characterize the underlying energy landscape that governs the reaction. The widely accepted two-state kinetic model for protein unfolding describes that the protein unfolds by crossing over a single energy barrier, with the implicit assumption of a single transition state and a well-defined activation energy barrier. Based on this assumption, the ensemble-averaged survival probability is expected to follow single exponential time dependence. However, it has become increasingly clear that the saddle point of the free-energy surface in most reactions is populated by ensembles of conformations, leading to nonexponential kinetics. Here we present a theory that generalizes the two-state model to include static disorder of conformational degrees of freedom to fully account for a diverse set of unfolding pathways. Using single-molecule force-clamp spectroscopy, we study the nonexponential kinetics of single ubiquitin proteins unfolding under constant forces. We find that the measured variance in the barrier heights has a quadratic dependence on force. Our study illustrates a novel adaptation of the classical Arrhenius equation that accounts for the microscopic origins of nonexponential kinetics. Our theory provides a direct approach in determining the variance in the barrier heights of a reaction. We extend our theoretical model to investigate the kinetics of two different reactions, protein unfolding and disulfide-bond reduction, both occurring within the same protein molecule. We measure the variance of the barrier heights, which quantifies the heterogeneity of the reaction pathway for both reactions. In contrast to protein unfolding, we find that the variance of the barrier heights for disulfide-bond reduction is close to zero, reflecting the differences between these two reactions. These results strongly suggest that the transition state for a disulfide-bond reduction is well defined, as opposed to protein unfolding. The Bell model assumes that the distance to the transition state is force independent. However, in many systems, it has been observed that the transition state moves toward the destabilized state upon perturbation. This effect, known as the Hammond effect, would predict that the distance to the transition state decreases with force. This hypothesis remains unexplored in protein unfolding under force. To elucidate the conformational plasticity of the transition state structure upon the application of force, we probe the unfolding kinetics of ubiquitin and NuG2 over a broad range of forces. We use the force-ramp assay to measure probability distribution of unfolding forces. Based on the standard two-state model, the force-dependent lifetimes can be obtained by transforming the probability distribution of unfolding forces. However, this formalism is invalid for proteins exhibiting the dispersed kinetics, as we observed in ubiquitin. By measuring the lifetimes over a wide range of forces, we discover that the distance to the transition state for NuG2 exhibits a weak force dependency. The measured value of the distance to the transition state is 0.22 nm, comparable to the size of a water molecule. The observed non-Hammond behavior revealed an integral structural role of water molecules bridging the unfolding transition state, constraining the movement of the unfolding transition state. Finally, in order to test the Kramers theory that would predict that the distance to the transition state continuously decreases with force, we explore the kinetics of disulfide bond reduction by hydroxide anions over a wide range of forces. On the contrary to the Kramers prediction, we observe that the reduction rate exhibits two distinct exponential dependencies on the pulling force, revealing a discontinuous shift in the distance to the transition state. The experimental data show that the distance to the transition state is ~ 0.5 Å in the low-force regime (< 500 pN), and changes to a much shorter value of ~ 0.1 Å in the high-force regime (> 500 pN). We propose a plausible molecular scenario that is consistent with our experimental results. We suggest that the substrate disulfide bond undergoes a conformational change under a stretching force above 500 pN. Our results show the first observation that the application of a mechanical force to the protein disulfide bond causes an abrupt change in reactivity.Biophysicstk2179PhysicsDissertationsThe Light Response of the XENON100 Time Projection Chamber and the Measurements of the Optical Parameters with the Xenon Scintillation Light
http://academiccommons.columbia.edu/catalog/ac:156958
Choi, BinWed, 20 Feb 2013 00:00:00 +0000The XENON program is a phased project using liquid xenon as a sensitive detector medium in search for weakly interacting massive particles (WIMPs). These particles are the leading candidates to explain the non-baryonic, cold dark matter in our Universe. XENON100, the successor experiment of XENON10, has increased the target liquid xenon mass to 61 kg with a 100 times reduction in background rate enabling a large increase in sensitivity to WIMP- nucleon interaction cross-section. To-date, the most stringent limit on this cross-section over a wide range of WIMP masses have been obtained with XENON100. XENON100 is a detector responding to the scintillation of xenon and the work of this thesis will mainly focus on the light response of the detector. Chapter 1 describes the evidences for dark matter and some of the detection methods, roughly divided by the indirect and the direct detection. In the section 1.2.2 for direct detection, a treatment of interaction rate of WIMPs is introduced. Chapter 2 is a description of the XENON100 detector, some of the main characteristics of liquid xenon, followed by the detector design. In Chapter 3, the light response of the XENON100 time projection chamber (TPC) is explained, including the Monte Carlo simulation work that was carried out prior to the main data taking. The Monte Carlo provided the basic idea of understanding the detector in the early stage of design and calibration, but the actual corrections of the light signals were determined later with the real data. Several optical parameters are critical in explaining the light response, such as the quantum efficiency (QE) of the photomultipliers (PMTs) used in the detector and the reflectivity of the teflon (Polytetrafluoroethylene, PTFE) material that surrounds the liquid xenon target volume and defines the TPC. Since the few existing measurements of reflectivity of PTFE in liquid xenon were performed in different conditions and thus could not be applied, the XENON collaboration put some effort in setting up a reliable and an independent measurement for these parameters. The QE of the Hamamatsu R8520 PMTs at liquid xenon temperature was measured at the Columbia Nevis Laboratory, as described in Chapter 4. A similar but a revised setup was built later at the University of Muenster in Germany for measuring the reflectivity of the PTFE (Chapter 5). These measurements are important for a deeper understanding of XENON100 and the next phase of the program with a XENON1T as well as for other liquid xenon experiments. Chapter 6 explains the details of the energy scale derived from the measurement of the light signals in XENON100 and the cuts used for the analysis, which has led to the most recent scientific results from this experiments. In 2012, the XENON100 dark matter results from 225 live days set the most stringent limit on the spin-independent elastic WIMP- nucleon interaction cross section for WIMP masses above 8 GeV/c 2, with a minimum of 2 × 10and minus;45 cm 2 at 55 GeV/c 2 and 90% confidence level. With this result XENON100 continues to be the leading experiment in the direct search for dark matterPhysicsbc2196PhysicsDissertationsXENON100 Dark Matter Search: Scintillation Response of Liquid Xenon to Electronic Recoils
http://academiccommons.columbia.edu/catalog/ac:156964
Lim, Kyungeunhttp://hdl.handle.net/10022/AC:P:19131Wed, 20 Feb 2013 00:00:00 +0000Dark matter is one of the missing pieces necessary to complete the puzzle of the universe. Numerous astrophysical observations at all scales suggest that 23 % of the universe is made of nonluminous, cold, collisionless, nonbaryonic, yet undiscovered dark matter. Weakly Interacting Massive Particles (WIMPs) are the most well-motivated dark matter candidates and significant efforts have been made to search for WIMPs. The XENON100 dark matter experiment is currently the most sensitive experiment in the global race for the first direct detection of WIMP dark matter. XENON100 is a dual-phase (liquid-gas) time projection chamber containing a total of 161 kg of liquid xenon (LXe) with a 62kg WIMP target mass. It has been built with radiopure materials to achieve an ultra-low electromagnetic background and operated at the Laboratori Nazionali del Gran Sasso in Italy. WIMPs are expected to scatter off xenon nuclei in the target volume. Simultaneous measurement of ionization and scintillation produced by nuclear recoils allows for the detection of WIMPs in XENON100. Data from the XENON100 experiment have resulted in the most stringent limits on the spin-independent elastic WIMP- nucleon scattering cross sections for most of the significant WIMP masses. As the experimental precision increases, a better understanding of the scintillation and ionization response of LXe to low energy (< 10 keV) particles is crucial for the interpretation of data from LXe based WIMP searches. A setup has been built and operated at Columbia University to measure the scintillation response of LXe to both electronic and nuclear recoils down to energies of a few keV, in particular for the XENON100 experiment. In this thesis, I present the research carried out in the context of the XENON100 dark matter search experiment. For the theoretical foundation of the XENON100 experiment, the first two chapters are dedicated to the motivation for and detection medium choice of the XENON100 experiment, respectively. A general review about dark matter focusing on WIMPs and their direct detection with liquid noble gas detectors is presented in Chap. 1. LXe as an attractive WIMP detection medium is explained in Chap. 2. The XENON100 detector design, the detector, and its subsystems are detailed in Chap. 3. The calibration of the detector and the characterized detector response used for the discrimination of a WIMP-like signal against background are explained in Chap. 4. In an effort to understand the background, anomalous electronic recoils were studied extensively and are described in Chap. 5. In order to obtain a better understanding of the electronic recoil background of XENON100, including an estimation of the electronic recoil background contribution, as well as to interpret dark matter results such as annual modulation, measurement of the scintillation yield of low-energy electrons in LXe was performed in 2011, with the dedicated setup mentioned above. The results from this measurement are discussed in Chap. 6. Finally, the results for the latest science data from XENON100 to search for WIMPs, comprising 225 live-days taken over 13 months during 2011 and 2012 are explained in Chap. 7.Physics, AstrophysicsPhysicsDissertationsQuantum Hall transport in graphene and its bilayer
http://academiccommons.columbia.edu/catalog/ac:156080
Zhao, Yuehttp://hdl.handle.net/10022/AC:P:18862Wed, 30 Jan 2013 00:00:00 +0000Graphene has generated great interest in the scientific community since its discovery because of the unique chiral nature of its carrier dynamics. In monolayer graphene, the relativistic Dirac spectrum for the carriers results in an unconventional integer quantum Hall effect, with a peculiar Landau Level at zero energy. In bilayer graphene, the Dirac-like quadratic energy spectrum leads to an equally interesting, novel integer quantum Hall effect, with a eight-fold degenerate zero energy Landau level. In this thesis, we present transport studies at high magnetic field on both monolayer and bilayer graphene, with a particular emphasis on the quantum Hall (QH) effect at the charge neutrality point, where both systems exhibit broken symmetry of the degenerate Landau level at zero energy. We also present data on quantum Hall edge transport across the interface of a graphene monolayer and bilayer junction, where peculiar edge state transport is observed. We investigate the quantum Hall effect near the charge neutrality point in bilayer graphene, under high magnetic fields of up to 35~T using electronic transport measurements. In the high field regime, we observe a complete lifting of the eight-fold degeneracy of the zero-energy Landau level, with new quantum Hall states corresponding to filling factors $\nu=0$, 1, 2 and 3. Measurements of the activation energy gap in tilted magnetic fields suggest that the Landau level splitting at the newly formed $\nu=$1, 2 and 3 filling factors does not exhibit low-energy spin flip excitation. These measurements are consistent with the formation of a quantum Hall ferromagnet. In addition, we observed insulating behavior in the two terminal resistance of the $\nu=$0 state at high fields. For monolayer graphene, we report on magneto-resistance measurements at the broken-symmetry of the zero-energy Landau level, using both a conventional two-terminal measurement of suspended graphene devices, which is sensitive to bulk and edge conductance, and a Corbino measurement on high mobility on-substrate devices, which is sensitive to the bulk conductance only. At $\nu=0$, we observe a vanishing conductance with increasing magnetic fields in both cases. By examining the resistance changes of this insulating state with varying perpendicular and in-plane fields, we probe the spin-active components of the excitations in total fields of up to 45 Tesla. Our results strongly suggest that the $\nu=0$ quantum Hall state in single layer graphene is not spin polarized, while a spin-polarized state with spin-flip excitations forms at $\nu=1$. For monolayer and bilayer graphene junction system, we first present a surface potential study across the monolayer/bilayer interface. Then we present experimental investigations of the edge state transition across the interface in the quantum Hall regime. Both monolayer graphene (MG) and bilayer graphene (BG) develop their own Landau levels under high magnetic field. While transport measurements show their distinct quantum Hall effects in the separate parts of the monolayer and bilayer respectively, the transport measurement across the interface exhibits unusual transverse transport behavior. The transverse resistance across the MG/BG interface is asymmetric for opposite sides of the Hall bar, and its polarity can be changed by reversing the magnetic field direction. When the quantum Hall plateaus of MG and BG overlap, quantized resistance appears only on one side of the Hall bar electrode pairs that sit across the junction. These experimental observations can be ascribed to QH edge state transport across the MG/BG interface. We also present sample fabrication details, particularly the efforts to eliminate mobility-limiting factors, including cleaning polymer residue from the electron beam lithography process via thermal annealing and removing/changing the substrate by suspending multi-probe graphene devices.Physicsyz2444PhysicsDissertationsSearching for Reactor Antineutrino Flavor Oscillations with the Double Chooz Far Detector
http://academiccommons.columbia.edu/catalog/ac:169558
Franke, ArthurWed, 23 Jan 2013 00:00:00 +0000This dissertation presents results from a search for reactor antineutrinoₑ flavor oscillations using the Double Chooz Far Detector. The search was performed by observing the rate and energy spectrum of antineutrinoₑ interacting via Inverse Beta Decay in a Gd-doped liquid scintillator detector, and comparing the observation to an expectation based on a prediction of the emitted reactor flux. The Columbia University neutrino group was instrumental in construction of the Double Chooz Outer Veto, as well as the analysis efforts leading to two oscillation measurement results. The most recent analysis is presented herein, focusing on 251.27 days of data (or 33.71 GW-ton-years of exposure). In these data, 8249 IBD candidates were observed, compared to a signal+background prediction of 8936.8. A fit to a two-neutrino oscillation model considering event rate, spectral shape, and time yields a best-fit value of sin² (2θ₁₃) = 0.109 ± 0.030 (stat.) ± 0.025 (syst.) at Δm 2/31 = 2.32 × 10⁻³ eV², with χ²_RS/d.o.f. = 42.1/35. A frequentist method deems the null-oscillation hypothesis excluded by the data at 99.8% C.L., or 2.9σ. These results are in agreement with the measurements of other modern reactor antineutrinoₑ experiments.Particle physicsajf2140PhysicsDissertationsExploring the String Landscape: The Dynamics, Statistics, and Cosmology of Parallel Worlds
http://academiccommons.columbia.edu/catalog/ac:155499
Ahlqvist, Stein Pontushttp://hdl.handle.net/10022/AC:P:15784Tue, 15 Jan 2013 00:00:00 +0000This dissertation explores various facets of the low-energy solutions in string theory known as the string landscape. Three separate questions are addressed - the tunneling dynamics between these vacua, the statistics of their location in moduli space, and the potential realization of slow-roll inflation in the flux potentials generated in string theory. We find that the tunneling transitions that occur between a certain class of supersymmetric vacua related to each other via monodromies around the conifold point are sensitive to the details of warping in the near-conifold regime. We also study the impact of warping on the distribution of vacua near the conifold and determine that while previous work has concluded that the conifold point acts as an accumulation point for vacua, warping highly dilutes the distribution in precisely this regime. Finally we investigate a novel form of inflation dubbed spiral inflation to see if it can be realized near the conifold point. We conclude that for our particular models, spiral inflation seems to rely on a de Sitter-like vacuum energy. As a result, whenever spiral inflation is realized, the inflation is actually driven by a vacuum energy.Physicsspa2111Physics, MathematicsDissertationsSimultaneous Immersion Mirau Interferometry
http://academiccommons.columbia.edu/catalog/ac:153355
Lyulko, Oleksandra V.http://hdl.handle.net/10022/AC:P:14948Mon, 15 Oct 2012 00:00:00 +0000The present work describes a novel imaging technique for label-free no-UV vibration-insensitive imaging of live cells in an epi-illumination geometry. This technique can be implemented in a variety of imaging applications. For example, it can be used for cell targeting as a part of a platform for targeted cell irradiations - single-cell microbeam. The goal of microbeam facilities is to provide biological researchers with tools to study the effects of ionizing radiation on live cells. A common way of cell labeling - fluorescent staining - may alter cellular metabolism and UV illumination presents potential damage for the genetic material. The new imaging technique will allow the researchers to separate radiation-induced effects from the effects caused by confounding factors like fluorescent staining or UV light. Geometry of irradiation endstations at some microbeam facilities precludes the use of transmitted light, e.g. in the Columbia University's Radiological Research Accelerator Facility microbeam endstation, where the ion beam exit window is located just below the sample. Imaging techniques used at such endstations must use epi-illumination. Mirau Interferometry is an epi-illumination, non-stain imaging modality suitable for implementation at a microbeam endstation. To facilitate interferometry and to maintain cell viability, it is desirable that cells stay in cell growth medium during the course of an experiment. To accommodate the use of medium, Immersion Mirau Interferometry has been developed. A custom attachment for a microscope objective has been designed and built for interferometric imaging with the possibility of immersion of the apparatus into cell medium. The implemented data collection algorithm is based on the principles of Phase-Shifting Interferometry. The largest limitation of Phase-Shifting Interferometry is its sensitivity to the vertical position of the sample. In environments where vibration isolation is difficult, this makes image acquisition challenging. This problem was resolved by integration of polarization optics into the optics of the attachment to enable simultaneous creation and spatial separation of two interferograms, which, combined with the background image, are used to reconstruct the intensity map of the specimen. Giving the name Simultaneous Immersion Mirau Interferometry to this approach, simultaneous acquisition of all interferograms per image has eliminated the issue of vibrations. The designed compound microscope attachment has been manufactured and tested; the system produces images of quality, sufficient to perform targeted cellular irradiation experiments.Physics, OpticsRadiation Oncology, Physics, Center for Radiological ResearchDissertationsAn Accelerator Measurement of Atomic X-ray Yields in Exotic Atoms and Implications for an Antideuteron-Based Dark Matter Search
http://academiccommons.columbia.edu/catalog/ac:166530
Aramaki, Tsuguohttp://hdl.handle.net/10022/AC:P:14873Wed, 10 Oct 2012 00:00:00 +0000The General AntiParticle Spectrometer (GAPS) is a novel approach for indirect dark mat- ter searches that exploits cosmic antideuterons. The low energy antideuteron provides a clean dark matter signature, since the antideuteron production by cosmic ray interactions is suppressed at low energy, while the WIMP-WIMP annihilation can produce low energy an- tideuterons. GAPS utilizes a distinctive detection method using atomic X-rays and charged particles from the exotic atom as well as the timing, stopping range and dE/dX energy deposit of the incoming particle, which provides excellent antideuteron identification. Prior to the future balloon experiment, an accelerator test was conducted in 2004 and 2005 at KEK, Japan to measure the atomic X-rays of antiprotonic exotic atoms produced by different targets. In 2005, solid targets were tested to avoid the bulky fixture of the gas target and also to have flexibility of the detector geometry in the flight experiment. Recently, we have developed a simple cascade model and the parameters were fitted with the experimental results. The cascade model was extended to the antideuteronic exotic atom for the GAPS flight experiment. GEANT4 simulation was conducted to obtain optimized cuts on the timing, stopping range, dE/dX energy deposit, atomic X-rays, and annihilation products, in order to eliminate the background. Based on the simulation results, we have estimated the GAPS sensitivity with the antideuteron flux. GAPS has a strong potential to detect a dark matter signature.Physics, Astrophysicsta2159PhysicsDissertationsSearch for Excited Randall-Sundrum Gravitons with Semi-Leptonic Diboson Final States in 4.7 fb-1 of Proton-Proton Collisions using the ATLAS Detector at the Large Hadron Collider
http://academiccommons.columbia.edu/catalog/ac:153207
Williams, Eric Lloydhttp://hdl.handle.net/10022/AC:P:14878Wed, 10 Oct 2012 00:00:00 +0000This dissertation describes a search for resonant WW and WZ production in the lvjj decay channel using 4.701 fb-1 of sqrt(s) = 7 TeV LHC collision data collected by the ATLAS detector. Events with a single charged lepton, at least two jets and missing transverse energy are analyzed and no significant deviation from the Standard Model prediction is observed. Upper limits on the production cross section are interpreted as lower limits on the mass of a resonance and are derived assuming two warped extra-dimension production modes: the original Randall-Sundrum (RS1) model and the more recent "bulk" Randal-Sundrum (Bulk RS) model. The mass range for both models is excluded at 95% CL with a lower mass limit for an RS1 graviton of 936 GeV and 714 GeV for the Bulk RS graviton.Particle physicselw2113Applied Physics and Applied Mathematics, PhysicsDissertationsSimulations of Dynamic Relativistic Magnetospheres
http://academiccommons.columbia.edu/catalog/ac:152158
Parfrey, Kyle Patrickhttp://hdl.handle.net/10022/AC:P:14563Wed, 29 Aug 2012 00:00:00 +0000Neutron stars and black holes are generally surrounded by magnetospheres of highly conducting plasma in which the magnetic flux density is so high that hydrodynamic forces are irrelevant. In this vanishing-inertia---or ultra-relativistic---limit, magnetohydrodynamics becomes force-free electrodynamics, a system of equations comprising only the magnetic and electric fields, and in which the plasma response is effected by a nonlinear current density term. In this dissertation I describe a new pseudospectral simulation code, designed for studying the dynamic magnetospheres of compact objects. A detailed description of the code and several numerical test problems are given. I first apply the code to the aligned rotator problem, in which a star with a dipole magnetic field is set rotating about its magnetic axis. The solution evolves to a steady state, which is nearly ideal and dissipationless everywhere except in a current sheet, or magnetic field discontinuity, at the equator, into which electromagnetic energy flows and is dissipated. Magnetars are believed to have twisted magnetospheres, due to internal magnetic evolution which deforms the crust, dragging the footpoints of external magnetic field lines. This twisting may be able to explain both magnetars' persistent hard X-ray emission and their energetic bursts and flares. Using the new code, I simulate the evolution of relativistic magnetospheres subjected to slow twisting through large angles. The field lines expand outward, forming a strong current layer; eventually the configuration loses equilibrium and a dynamic rearrangement occurs, involving large-scale rapid magnetic reconnection and dissipation of the free energy of the twisted magnetic field. When the star is rotating, the magnetospheric twisting leads to a large increase in the stellar spin-down rate, which may take place on the long twisting timescale or in brief explosive events, depending on where the twisting is applied and the history of the system. One such explosive field-expansion and reconnection event may have been responsible for the 27 August 1998 giant flare from SGR 1900+14, and the coincident sudden increase in spin period, or "braking glitch." The inner magnetospheres of relativistic compact objects are in strongly curved spacetimes. I describe the extension of the code to general-relativistic simulations, including the hypersurface foliation method and the 3+1 equations of force-free electrodynamics in curved, evolving spacetimes. A simple test problem for dynamical behavior in the Schwarzschild metric is presented, and the evolutions of the magnetospheres surrounding neutron stars and black holes, in vacuum and in force-free plasma, are compared.Astrophysicskp2226Physics, Astronomy and AstrophysicsDissertationsThe Future of Learned Societies
http://academiccommons.columbia.edu/catalog/ac:150296
O'Donnell, James; Weinberg, Erick J.; Rittenberg, Stephen A.http://hdl.handle.net/10022/AC:P:14146Thu, 19 Jul 2012 00:00:00 +0000James O'Donnell, Georgetown University Provost and Vice President for Publications of the American Philological Association, discusses the future of scholarly publishing and learned societies. Columbia University Physics professor Erick Weinberg, editor of the American Physical Society journal Physical Review D, responds. Learned societies have published the work of their members for over three hundred years and do it very well. Now, we are surrounded with huge quantities of digital information, and societies are challenged to consider afresh their motives, means, and opportunities. What should scholars do?Information science, Higher educationejw2, sar3Physics, Scholarly Communication Program, Center for Digital Research and Scholarship, Office of the Provost, Libraries and Information ServicesInterviews and roundtablesJet Quenching in Relativistic Heavy Ion Collisions at the LHC
http://academiccommons.columbia.edu/catalog/ac:147698
Angerami, Aaronhttp://hdl.handle.net/10022/AC:P:13442Thu, 07 Jun 2012 00:00:00 +0000Jet production in relativistic heavy ion collisions is studied using Pb+Pb collisions at a center of mass energy of 2.76 TeV per nucleon. The measurements reported here utilize data collected with the ATLAS detector at the LHC from the 2010 Pb ion run corresponding to a total integrated luminosity of 7 µ b^(-1). The results are obtained using fully reconstructed jets using the anti-k t algorithm with a per-event background subtraction procedure. A centrality-dependent modification of the dijet asymmetry distribution is observed, which indicates a higher rate of asymmetric dijet pairs in central collisions relative to periphal and pp collisions. Simultaneously the dijet angular correlations show almost no centrality dependence. These results provide the first direct observation of jet quenching. Measurements of the single inclusive jet spectrum, measured with jet radius parameters R=0.2, 0.3, 0.4 and 0.5, are also presented. The spectra are unfolded to correct for the finite energy resolution introduced by both detector effects and underlying event fluctuations. Single jet production, through the central-to-peripheral ratio R CP, is found to be suppressed in central collisions by approximately a factor of two, nearly independent of the jet p T. The R CP is found to have a small but significant increase with increasing R, which may relate directly to aspects of radiative energy loss.Physics, Nuclear physics, Particle physicsara2014PhysicsDissertationsCluster Dynamical Mean-Field Theory: Applications to High-Tc Cuprates and to Quantum Chemistry
http://academiccommons.columbia.edu/catalog/ac:147701
Lin, Nanhttp://hdl.handle.net/10022/AC:P:13443Thu, 07 Jun 2012 00:00:00 +0000In this thesis we use the recently developed dynamical mean-field approximation to study problems in strongly correlated electron systems, including high-Tc cuprate superconductors as well as a few quantum chemical reference systems. We start with an introduction to the background of the interacting electron systems, followed by a brief description on the current understanding of the physics of high-Tc cuprate superconductors. The approximate models that enter the theoretical framework will be discussed afterwards. Some quantum chemical methods for many-body quantum systems are included for review. Next we present the numerical methods employed in our study. The formalism of the dynamical mean-field approximation will be introduced including the single-site and cluster versions, followed by the Exact Diagonalization impurity solver for the solution of the quantum impurity model. Maximum Entropy analytic continuation method is also discussed, which is useful to obtain the physically relevant response functions. Then we apply dynamical mean-field approximation to high-Tc cuprate superconductors. The two-particle response functions, such as Raman scattering intensity and optical conductivity, are computed for the two dimensional Hubbard model. The calculations include the vertex corrections which are essential to obtain physically reasonable results in interacting electron systems. We also study the physics of the pseudogap in cuprates. The suppression of density of states near Fermi surface is present in our calculations, which is in qualitative agreement with the experimental data. Finally we discuss the application of dynamical mean-field theory to quantum chemistry. We extend the formalism of dynamical mean-field approximation to finite systems, and compare its performance in hydrogen clusters with different spatial configurations to other leading quantum chemical approaches. Dynamical mean-field theory involves mapping onto a quantum impurity model. We further examine the quantum impurity model representation of the transition metal dioxide molecules. The conceptual and technical difficulties will be discussed.Condensed matter physicsnl2219Physics, ChemistryDissertationsSearch for gravitons using merged jets from Z boson decays with the ATLAS experiment
http://academiccommons.columbia.edu/catalog/ac:146686
Penson, Alexander Vincenthttp://hdl.handle.net/10022/AC:P:13154Mon, 07 May 2012 00:00:00 +0000A search is presented for anomalous production of a pair of gauge bosons (ZZ or WZ) from the decay of a narrow massive resonance. Data corresponding to 2.0 fb-1 of integrated luminosity collected by the ATLAS experiment from proton-proton collisions at 7 TeV. Events with two charged leptons and either two resolved jets or one merged jet are analyzed and found to be consistent with the Standard Model background expectation. In the absence of an excess, lower limits on the mass of a resonance are set using the original Randall-Sundrum (RS1) model as a benchmark. The observed (expected) lower limit on the mass of an excited graviton decaying to ZZ is 870 (950) GeV at 95% confidence level. Limits are also set on a more recent version of the Randall-Sundrum model where Standard Model particles are allowed to propagate in the five dimensional bulk. An excited graviton in this model is excluded for masses between 500 and 630 GeV.Physicsavp2106PhysicsDissertationsUsing machine learning to predict gene expression and discover sequence motifs
http://academiccommons.columbia.edu/catalog/ac:146375
Li, Xuejinghttp://hdl.handle.net/10022/AC:P:13057Mon, 30 Apr 2012 00:00:00 +0000Recently, large amounts of experimental data for complex biological systems have become available. We use tools and algorithms from machine learning to build data-driven predictive models. We first present a novel algorithm to discover gene sequence motifs associated with temporal expression patterns of genes. Our algorithm, which is based on partial least squares (PLS) regression, is able to directly model the flow of information, from gene sequence to gene expression, to learn cis regulatory motifs and characterize associated gene expression patterns. Our algorithm outperforms traditional computational methods e.g. clustering in motif discovery. We then present a study of extending a machine learning model for transcriptional regulation predictive of genetic regulatory response to Caenorhabditis elegans. We show meaningful results both in terms of prediction accuracy on the test experiments and biological information extracted from the regulatory program. The model discovers DNA binding sites ab intio. We also present a case study where we detect a signal of lineage-specific regulation. Finally we present a comparative study on learning predictive models for motif discovery, based on different boosting algorithms: Adaptive Boosting (AdaBoost), Linear Programming Boosting (LPBoost) and Totally Corrective Boosting (TotalBoost). We evaluate and compare the performance of the three boosting algorithms via both statistical and biological validation, for hypoxia response in Saccharomyces cerevisiae.Physicsxl2118PhysicsDissertationsKaon to two pions decays from lattice QCD: ΔI=1/2 rule and CP violation
http://academiccommons.columbia.edu/catalog/ac:146416
Liu, Qihttp://hdl.handle.net/10022/AC:P:13068Mon, 30 Apr 2012 00:00:00 +0000We report a direct lattice calculation of the K to ππ decay matrix elements for both the ΔI = 1/2 and 3/2 amplitudes A0 and A2 on a 2+1 flavor, domain wall fermion, 163 × 32 × 16 lattice ensemble and a 243 × 64 × 16 lattice ensemble. This is a complete calculation in which all contractions for the required ten, four-quark operators are evaluated, including the disconnected graphs in which no quark line connects the initial kaon and final two-pion states. These lattice operators are nonperturbatively renormalized using the Rome-Southampton method and the quadratic divergences are studied and removed. This is an important but notoriously difficult calculation, requiring high statistics on a large volume. In this work we take a major step towards the computation of the physical K → ππ amplitudes by performing a complete calculation at unphysical kinematics with pions of mass 422MeV and 329MeV at rest in the kaon rest frame. With this simplification we are able to resolve Re(A0) from zero for the first time, with a 25% statistical error on the 163 lattice and 15% on the 243 lattice. The complex amplitude A2 is calculated with small statistical errors. We obtain the ΔI = 1/2 rule with an enhancement factor of 9.1(21) and 12.0(17) on these two ensembles. From the detailed analysis of the results we gain a deeper understanding of the origin of the ΔI = 1/2 rule. We also calculate the complex amplitude A0, a calculation central to understanding and testing the standard model of CP violation in the kaon system. The final result for the measure of direct CP violation, ε′, calculated at unphysical kinematics has an order of 100% statistical error, so this only serves as an order of magnitude check.Theoretical physicsql2142PhysicsDissertationsInhibition stabilized network model in the primary visual cortex
http://academiccommons.columbia.edu/catalog/ac:144745
Zhao, Junhttp://hdl.handle.net/10022/AC:P:12617Fri, 17 Feb 2012 00:00:00 +0000In this paper, we studied neural networks of both excitatory and inhibitory populations with inhibition stabilized network (ISN) models. In ISN models, the recurrent excitatory connections are so strong that the excitatory sub-network is unstable if the inhibitory firing rate is fixed; however, the entire network is stable due to inhibitory connections. In such networks, external input to inhibitory neurons reduced their responses due to the withdrawal of network excitation (Tsodyks et al., 1997). This paradoxical effect of the ISN was observed in recent surround suppression experiments in the primary visual cortex with direct membrane conductance measurements (Ozeki et al., 2009). In our work, we used a linearized rate model of both excitatory and inhibitory populations with weight matrices dependent on the locations of the neurons. We applied this model to study surround suppression effects and searched for networks with appropriated parameters. The same model was also applied in the study of spontaneous activities in awake ferrets. Both studies led to network solutions in the ISN regime, suggesting that ISN mechanisms might play an important role in the neural circuitry in the primary visual cortex.Neurosciences, Biophysicsjz2110PhysicsDissertationsDirect Jet Reconstruction in Proton-Proton and Copper-Copper Collisions at √sNN = 200 GeV
http://academiccommons.columbia.edu/catalog/ac:144594
Lai, Yue Shihttp://hdl.handle.net/10022/AC:P:12560Wed, 15 Feb 2012 00:00:00 +0000Collision of heavy nuclei at the Relativistic Heavy Ion Collider (RHIC) recreates the state of high temperature quark-gluon plasma that existed shortly after the Big Bang. Measurement using single particle spectra and two-particle correlation shows that this medium is largely opaque to the transit of a high energy quark or gluon. Reconstructing the kinematics of these quarks and gluons can provide additional constraints for the property of their interaction with the medium. While the direct reconstruction of quantum chromodynamics jets, the final state showers of quarks and gluons, has become an indispensable tool at hadron and electron accelerator experiments, the application of this technique to heavy ion collisions at the RHIC energy has been considered a hard problem. The relatively low yield of high transverse momentum jets would have to be detected within a large, fluctuating background that can give rise to a false jet signal. At the RHIC PHENIX experiment, jet reconstruction also has to cope with the limited aperture of the central arm spectrometers. To overcome both problems, which can distort the jet signal in the traditional reconstruction algorithms, this thesis develops an algorithm that reconstructs the jets as maxima of the Gaussian filtered event transverse momentum distribution. The Gaussian angular weighting causes the algorithm to become more sensitive to the jet core versus the jet periphery. It is then combined with a fake jet rejection discriminant to remove the background fluctuation from the jet signal. This algorithm is used to obtain the first jet measurement in heavy ion environment at PHENIX, using data from the 2004/2005 RHIC run. The result includes the proton-proton inclusive jet spectrum, the proton-proton fragmentation function, the copper-copper jet nuclear modification factor, the copper-copper jet central-to-peripheral modification factor, and the copper-copper dijet azimuthal correlation. The measured copper-copper jet nuclear modification factor shows that there is a significant initial state effect to the jet suppression. The observation of no broadening in the copper-copper dijet azimuthal correlation indicates that the traditional energy loss picture via multiple soft scattering may not be applicable to the quark-gluon plasma.Nuclear physicsPhysicsDissertationsThe XENON100 Dark Matter Experiment: Design, Construction, Calibration and 2010 Search Results with Improved Measurement of the Scintillation Response of Liquid Xenon to Low-Energy Nuclear Recoils
http://academiccommons.columbia.edu/catalog/ac:143844
Plante, GuillaumeFri, 27 Jan 2012 00:00:00 +0000An impressive array of astrophysical observations suggest that 83% of the matter in the universe is in a form of non-luminous, cold, collisionless, non-baryonic dark matter. Several extensions of the Standard Model of particle physics aimed at solving the hierarchy problem predict stable weakly interacting massive particles (WIMPs) that could naturally have the right cosmological relic abundance today to compose most of the dark matter if their interactions with normal matter are on the order of a weak scale cross section. These candidates also have the added benefit that their properties and interaction rates can be computed in a well defined particle physics model. A considerable experimental effort is currently under way to uncover the nature of dark matter. One method of detecting WIMP dark matter is to look for its interactions in terrestrial detectors where it is expected to scatter off nuclei. In 2007, the XENON10 experiment took the lead over the most sensitive direct detection dark matter search in operation, the CDMS II experiment, by probing spin-independent WIMP-nucleon interaction cross sections down to σχN ~ 5 × 10-44 cm2 at 30GeV/c2. Liquefied noble gas detectors are now among the technologies at the forefront of direct detection experiments. Liquid xenon (LXe), in particular, is a well suited target for WIMP direct detection. It is easily scalable to larger target masses, allows discrimination between nuclear recoils and electronic recoils, and has an excellent stopping power to shield against external backgrounds. A particle losing energy in LXe creates both ionization electrons and scintillation light. In a dual-phase LXe time projection chamber (TPC) the ionization electrons are drifted and extracted into the gas phase where they are accelerated to amplify the charge signal into a proportional scintillation signal. These two signals allow the three-dimensional localization of events with millimeter precision and the ability to fiducialize the target volume, yielding an inner core with a very low background. Additionally, the ratio of ionization and scintillation can be used to discriminate between nuclear recoils, from neutrons or WIMPs, and electronic recoils, from γ or β backgrounds. In these detectors, the energy scale is based on the scintillation signal of nuclear recoils and consequently the precise knowledge of the scintillation efficiency of nuclear recoils in LXe is of prime importance. Inspired by the success of the XENON10 experiment, the XENON collaboration designed and built a new, ten times larger, with a one hundred times lower background, LXe TPC to search for dark matter. It is currently the most sensitive direct detection experiment in operation. In order to shed light on the response of LXe to low energy nuclear recoils a new single phase detector designed specifically for the measurement of the scintillation efficiency of nuclear recoils was also built. In 2011, the XENON100 dark matter results from 100 live days set the most stringent limit on the spin-independent WIMP-nucleon interaction cross section over a wide range of masses, down to σχN ~ 7 x 10-45 cm2 at 50GeV/c2, almost an order of magnitude improvement over XENON10 in less than four years. This thesis describes the research conducted in the context of the XENON100 dark matter search experiment. I describe the initial simulation results and ideas that influenced the design of the XENON100 detector, the construction and assembly steps that lead into its concrete realization, the detector and its subsystems, a subset of the calibration results of the detector, and finally dark matter exclusion limits. I also describe in detail the new improved measurement of the important quantity for the interpretation of results from LXe dark matter searches, the scintillation efficiency of low-energy nuclear recoils in LXe.Physics, Astrophysicsgp2135Physics, Astronomy and AstrophysicsDissertationsQuantum transport in graphene heterostructures
http://academiccommons.columbia.edu/catalog/ac:143085
Young, Andrea Franchinihttp://hdl.handle.net/10022/AC:P:12169Tue, 10 Jan 2012 00:00:00 +0000The two dimensional charge carriers in mono- and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. This thesis describes low temperature transport experiments designed to probe the consequences of this basic fact. The first part concerns the effect of the lattice pseudospin, an analog of a relativistic electron spin, on the scattering properties of mono- and bilayer graphene. We fabricate graphene devices with an extremely narrow local gates, and study ballistic carrier transport through the resulting barrier. By analyzing the interference of quasiparticles confined to the region beneath the gate, we are able to determine that charge carriers normally incident to the barrier are transmitted perfectly, a solid state analog of the Klein tunneling of relativistic quantum mechanics. The second part of the work describes the development of hexagonal boron nitride (hBN), an insulating isomorph of graphite, as a substrate and gate dielectric for graphene electronics. We use the enhanced mobility of electrons in h-BN supported graphene to investigate the effect of electronic interactions. We find interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of quantum Hall isospin ferromagnetic (QHIFM) states, which we study using tilted field magnetotransport. At yet higher fields, we observe fractional quantum Hall states which show signatures of the unique symmetries and anisotropies of the graphene QHIFM. The final part of the thesis details a proposal and preliminary experiments to probe isospin ordering in bilayer graphene using capacitance measurements.Condensed matter physicsafy2003PhysicsDissertationsSpectroscopy of Two Dimensional Electron Systems Comprising Exotic Quasiparticles
http://academiccommons.columbia.edu/catalog/ac:143043
Rhone, Trevor David Nathanielhttp://hdl.handle.net/10022/AC:P:12155Tue, 10 Jan 2012 00:00:00 +0000In this dissertation I present inelastic and elastic light scattering studies of collective states emerging from interactions in electron systems confined to two dimensions. These studies span the first, second and third Landau levels. I report for the first time, high energy excitations of composite fermions in the quantum fluid at v = 1/3. The high energies discovered represent excitations across multiple composite fermion energy levels, demonstrating the topological robustness of the fractional quantum Hall state at v = 1/3. This study sets the ground work for similar measurements of states in the second Landau level, such as those at v = 5/2. I present the first light scattering studies of low energy excitations of quantum fluids in the second Landau level. The study of low energy excitations of the quantum fluid at 3 ≥ v ≥ 5/2 reveals a rapid loss of spin polarization for v ≤ 3, as monitored by the intensity of the spin wave excitation at the Zeeman energy. The emergence of a continuum of low-lying excitations for v ≤ 3 reveals competing quantum phases in the second Landau level with intriguing roles of spin degrees of freedom and phase inhomogeneity. The first light scattering studies of the electron systems in the third Landau level are reported here. Measurements of low energy excitations and their spin degrees of freedom reveal contrasting behavior of states in the second and third Landau levels. I discuss these measurements in the context of the charge density wave phases, that are believed, by some, to dominate the third Landau level, and suggest ways of verifying this belief using light scattering. Distinct behavior in the dispersion of the spin wave at v = 3 is measured for the first time. The study may highlight differences in the first and second Landau levels that are manifested through the electron wavefunctions. In addition to intra-Landau level measurements, inter-Landau level studies are also reported. The results of which reveal roles of spin degrees of freedom and many body interactions in odd denominator integer quantum Hall states.Condensed matter physics, Optics, Quantum physicstnr2103Applied Physics and Applied Mathematics, PhysicsDissertationsProperties of Fragmentation Photons in p+p Collisions at 200 GeV Center-of-Mass Energy
http://academiccommons.columbia.edu/catalog/ac:142601
Hanks, Janette Alicehttp://hdl.handle.net/10022/AC:P:11852Wed, 30 Nov 2011 00:00:00 +0000The strong modification to the production of final state hadrons in heavy ion collisions is a key signature of the hot dense medium produced at energies achieved at the Relativistic Heavy Ion Collider (RHIC). Understanding the mechanisms for the parton energy loss responsible for these modifications is challenging and difficult to constrain with straightforward hadronic measurements, making it necessary to turn to more discriminating probes. One example of such a probe is photons produced by partons as they fragment, fragmentation photons, because the production mechanisms for such photons are similar to those for hadrons, but once produced, fragmentation photons will not interact directly with the medium. The challenge of distinguishing the signal for such jet-associate photons out of the large decay background motivates first making such measurements in the simple p + p environment. Combining data collected by the PHENIX detector during 2005 and 2006, the yield for fragmentation photons was measured to be on the order of several percent of all photons measured in association with a hadron with transverse momentum between 2 and 5 GeV/c. The use of two-particle correlations coupled with a sophisticated method for identifying and removing decay photons has made it possible to further study the jet properties of these fragmentation photons, in the form of pout and root mean square jT. These results will help to constrain both the underlying theoretical description of direct photon production in p + p, and modifications expected in heavy ion collisions.Physics, Nuclear physics, Particle physicsPhysicsDissertationsSpinning Black Hole Pairs: Dynamics and Gravitational Waves
http://academiccommons.columbia.edu/catalog/ac:141916
Grossman, Rebecca I.http://hdl.handle.net/10022/AC:P:11791Fri, 11 Nov 2011 00:00:00 +0000Black hole binaries will be an important source of gravitational radiation for both ground-based and future space-based gravitational wave detectors. The study of such systems will offer a unique opportunity to test the dynamical predictions of general relativity when gravity is very strong. To date, most investigations of black hole binary dynamics have focused attention on restricted scenarios in which the black holes do not spin (and thus are confined to move in a plane) and/or in which they stay on quasi-circular orbits. However, spinning black hole pairs in eccentric orbits are now understood to be astrophysically equally important. These spinning binaries exhibit a range of complicated dynamical behaviors, even in the absence of radiation reaction. Their conservative dynamics is complicated by extreme perihelion precession compounded by spin-induced precession. Although the motion seems to defy simple decoding, we are able to quantitatively define and describe the fully three-dimensional motion of arbitrary mass-ratio binaries with at least one black hole spinning and expose an underlying simplicity. To do so, we untangle the dynamics by constructing an instantaneous orbital plane and showing that the motion captured in that plane obeys elegant topological rules. In this thesis, we apply the above prescription to two formal systems used to model black hole binaries. The first is defined by the conservative 3PN Hamiltonian plus spin-orbit coupling and is particularly suitable to comparable-mass binaries. The second is defined by geodesics of the Kerr metric and is used exclusively for extreme mass-ratio binaries. In both systems, we define a complete taxonomy for fully three-dimensional orbits. More than just a naming system, the taxonomy provides unambiguous and quantitative descriptions of the orbits, including a determination of the zoom-whirliness of any given orbit. Through a correspondence with the rational numbers, we are able to show that all of the qualitative features of the well-studied equatorial geodesic motion around Schwarzschild and Kerr black holes are also present in more general black hole binary systems. This includes so-called zoom-whirl behavior, which turns out to be unexpectedly prevalent in comparable-mass binaries in the strong-field regime just as it is for extreme mass-ratio binaries. In each case we begin by thoroughly cataloging the constant radius orbits which generally lie on the surface of a sphere and have acquired the name "spherical orbits". The spherical orbits are significant as they energetically frame the distribution of all orbits. In addition, each unstable spherical orbit is asymptotically approached by an orbit that whirls an infinite number of times, known as a homoclinic orbit. We further catalog the homoclinic trajectories, each of which is the infinite whirl limit of some part of the zoom-whirl spectrum and has a further significance as the separatrix between inspiral and plunge for eccentric orbits. We then show that there exists a discrete set of orbits that are geometrically closed n-leaf clovers in a precessing orbital plane. When viewed in the full three dimensions, these orbits do not close, but they are nonetheless periodic when projected into the orbital plane. Each n-leaf clover is associated with a rational number, q, that measures the degree of perihelion precession in the precessing orbital plane. The rational number q varies monotonically with the orbital energy and with the orbital eccentricity. Since any bound orbit can be approximated as near one of these periodic n-leaf clovers, this special set offers a skeleton that illuminates the structure of all bound orbits in both systems, in or out of the equatorial plane. A first significant conclusion that can be drawn from this analysis is that all generic orbits in the final stages of inspiral under gravitational radiation losses are characterized by precessing clovers with few leaves, and that no orbit will behave like the tightly precessing ellipse of Mercury. We close with a practical application of our taxonomy beyond the illumination of conservative dynamics. The numerical calculation of the first-order (adiabatic) approximation to radiatively evolving inspiral motion in extreme mass-ratio binaries is currently hindered by prohibitive computational cost. Motivated by this limitation, we explain how a judicious use of periodic orbits can dramatically expedite both that calculation and the generation of snapshot gravitational waves from geodesic sources.Physics, Astrophysicsrg420Physics, Physics and Astronomy (Barnard College)DissertationsResults from the QUIET Q-Band Observing Season
http://academiccommons.columbia.edu/catalog/ac:141910
Dumoulin, Robert Nicolashttp://hdl.handle.net/10022/AC:P:11789Fri, 11 Nov 2011 00:00:00 +0000The Q/U Imaging ExperimenT (QUIET) is a ground-based telescope located in the high Atacama Desert in Chile, and is designed to measure the polarization of the Cosmic Microwave Background (CMB) in the Q and W frequency bands (43 and 95 GHz respectively) using coherent polarimeters. From 2008 October to 2010 December, data from more than 10,000 observing hours were collected, first with the Q-band receiver (2008 October to 2009 June) and then with the W-band receiver (until the end of the 2010 observing season). The QUIET data analysis effort uses two independent pipelines, one consisting of a maximum likelihood framework and the other consisting of a pseudo-C` framework. Both pipelines employ blind analysis methods, and each provides analysis of the data using large suites of null tests specific to the pipeline. Analysis of the Q-band receiver data was completed in November of 2010, confirming the only previous detection of the first acoustic peak of the EE power spectrum and setting competitive limits on the scalar-totensor ratio, r. In this dissertation, the results from the Q-band observing season using the maximum likelihood pipeline will be presented.Physics, AstrophysicsPhysicsDissertationsHEFT measurement of the hard X-ray size of the Crab Nebula and the hard X-ray optics of the Nuclear Spectroscopic Telescope Array (NuSTAR)
http://academiccommons.columbia.edu/catalog/ac:141649
An, Hongjunhttp://hdl.handle.net/10022/AC:P:11786Wed, 09 Nov 2011 00:00:00 +0000In this thesis, I discuss two topics: The High Energy Focusing Telescope (HEFT) and the Nuclear Spectroscopic Telescope Array (NuSTAR). HEFT is the first experiment done with imaging telescopes in the hard X-ray energy band (~20-70 keV). I briefly describe the instrument and the balloon campaign. The inflight calibration of the Point Spread Function (PSF) is done with a point source observation (~50 minutes of Cyg X-1 observation). With the PSF calibrated, I attempt to measuring the size of the Crab Nebula in this energy band. Analysis for aspect reconstruction, optical axis determination and the size measurement are described in detail. The size of the Crab Nebula is energy dependent due to synchrotron burn-off. The measurement of the size at different energies can provide us with important parameters for the pulsar wind nebula (PWN) model such as the magnetization parameter. With ~60 minutes of observation of the Crab Nebula with HEFT, I measure the size of the Crab Nebula at energies of 25-58 keV. The analysis technique I used for the size measurement here can be used for measuring the size of astrophysical objects whose sizes are comparable to the width of the PSF. NuSTAR is a satellite version of the HEFT experiment although the spatial and spectral resolution of the optics are improved significantly. And thus, the fabrication technique for the HEFT optics needed to be modified. I describe the fabrication technique for the NuSTAR optics, focusing on the epoxy selection and process development and the metrology systems for characterizing the figure of the glass surfaces.Physics, Astrophysicsha2153PhysicsDissertationsUnintegrated Gluon Distributions at Small-x
http://academiccommons.columbia.edu/catalog/ac:139284
Dominguez, Fabiohttp://hdl.handle.net/10022/AC:P:11293Wed, 28 Sep 2011 00:00:00 +0000The study of strong interactions at very high energies has prompted a large interest in the small-x regime of quantum chromodynamics where partons carry a small fraction of the momentum of their parent hadrons. In this regime gluon occupation numbers are believed to be very high leading to saturation of the corresponding parton densities. This thesis is intended to explore the validity of factorization approaches in the small-x regime and establish a relation with partonic interpretations when possible. Two fundamental unintegrated (transverse momentum dependent) gluon distributions are proposed as fundamental building blocks to describe all processes sensitive to the small-x regime which admit a factorized description. Single-particle production processes and two-particle production processes are studied in asymmetric collisions of a dilute probe scattering from a dense target and it is shown that it is possible to recover factorized expression in a particular kinematical limit.Nuclear physicsfad2111PhysicsDissertationsQuantum Chromodynamics with Eight and Twelve Degenerate Quark Flavors on the Lattice
http://academiccommons.columbia.edu/catalog/ac:137844
Jin, Xiao-Yonghttp://hdl.handle.net/10022/AC:P:11014Mon, 29 Aug 2011 00:00:00 +0000This thesis is concerned with the behavior of non-abelian gauge theories with many flavors of fermions. In perturbation theory, an infrared fixed point is predicted to exist, and theories become conformal in the low energy limit, in non-abelian gauge theories with the number of fermions just below the threshold of losing asymptotic freedom. With the number of fermion flavors even smaller than the number required for conformal behavior, the coupling constant is expected to run slowly or "walk". However, the exact number of fermion flavors that is required for the conformal behavior is unknown. This thesis probes for non-perturbative evidence for such behavior by simulating SU(3) gauge theories on the lattice with eight and twelve degenerate fermions in the fundamental representation. The naive staggered fermion action with the DBW2 gauge action is used in the simulations. The exact RHMC algorithm with the Omelyan integrator is used for simulating all eight-flavor gauge configurations and twelve-flavor gauge configurations with large masses, mq ≥ 0.01. For the other twelve-flavor simulations with smaller masses, mq < 0.01, the exact HMC algorithm with multiple mass preconditioning and the force gradient integrator is used. Comparisons are also done with previous simulations, which used the Wilson plaquette gauge action and the inexact R algorithm. Both zero temperature (Nt = 32) and finite temperature physics are studied in this thesis. For system with eight flavors, the focus of the zero temperature simulations is on three values of input couplings β = 0.54, 0.56 and 0.58, with two or three quark masses for each coupling value. The zero-temperature, lattice artifact bulk transition found with the Wilson plaquette action in becomes a rapid cross-over with the DBW2 gauge action. At finite temperatures, a first order phase transition is observed at the strongest coupling, β = 0.54. For systems with twelve flavors, a large amount of simulation is done at values of input couplings from β = 0.45 to 0.50. A zero-temperature bulk transition is found with quark masses mq = 0.006 and 0.008, and it ends in a second order critical point at masses slightly larger than 0.008. The system shows a mass-dependent rapid cross-over with quark masses mq ≥ 0.01 around the lattice couplings from β = 0.46 to β = 0.48. A finite temperature study at β = 0.49 shows a drastic change of behavior in the screening masses and other observables, which suggests the existence of a finite temperature >transition. All the evidences gathered in this thesis support the argument that theories of both eight and twelve flavors of fermion in the fundamental representation of SU(3) gauge group are consistent with the behavior one would expected from a theory with spontaneously broken chiral symmetry. The strongest supporting evidence is the linearity of mπ2 ∝ mq at zero temperatures and the existence of a chiral symmetry restoring transition at finite temperatures. We note that other lattice simulations, also exploring the hadronic observables, arrive at a similar conclusion, while simulations of the running of the coupling have claimed that the 12 flavor theory is conformal.Physicsxj2106PhysicsDissertationsFrom Measure Zero to Measure Hero: Periodic Kerr Orbits and Gravitational Wave Physics
http://academiccommons.columbia.edu/catalog/ac:137822
Perez-Giz, Gabehttp://hdl.handle.net/10022/AC:P:11008Mon, 29 Aug 2011 00:00:00 +0000A direct observational detection of gravitational waves - perhaps the most fundamental prediction of a theory of curved spacetime - looms close at hand. Stellar mass compact objects spiraling into supermassive black holes have received particular attention as sources of gravitational waves detectable by space-based gravitational wave observatories. A well-established approach models such an extreme mass ratio inspirals (EMRI) as an adiabatic progression through a series of Kerr geodesics. Thus, the direct detection of gravitational radiation from EMRIs and the extraction of astrophysical information from those waveforms require a thorough knowledge of the underlying geodesic dynamics. This dissertation adopts a dynamical systems approach to the study of Kerr orbits, beginning with equatorial orbits. We deduce a topological taxonomy of orbits that hinges on a correspondence between periodic orbits and rational numbers. The taxonomy defines the entire dynamics, including aperiodic motion, since every orbit is in or near the periodic set. A remarkable implication of this periodic orbit taxonomy is that the simple precessing ellipse familiar from planetary orbits is not allowed in the strong-field regime. Instead, eccentric orbits trace out precessions of multi-leaf clovers in the final stages of inspiral. Furthermore, for any black hole, there is some orbital angular momentum value in the strong-field regime below which zoom-whirl behavior becomes unavoidable. We then generalize the taxonomy to help identify nonequatorial orbits whose radial and polar frequencies are rationally related, or in resonance. The thesis culminates by describing how those resonant orbits can be leveraged for an order of magnitude or more reduction in the computational cost of adiabatic order EMRI trajectories, which are so prohibitively expensive that no such relativistically correct inspirals have been generated to date.Physics, Astrophysics, Applied mathematicsgep1PhysicsDissertationsAccretion topics in astrophysics
http://academiccommons.columbia.edu/catalog/ac:137835
Zalamea, Ivanhttp://hdl.handle.net/10022/AC:P:11012Mon, 29 Aug 2011 00:00:00 +0000Accretion theory is essential for understanding a multitude of varied astronomical observations such as X-ray binaries, active galactic nuclei and gamma-ray bursts. In this document three works on accretion will be presented.The first one is on the structure of an inviscid accretion disc with small angular momentum around a rotating black hole. This regime of accretion may occur in X-ray binaries and GRBs. The second work is on neutrino antineutrino annihilation in the vicinity of a hyper-accreting black hole. This work is relevant for the study of GRBs, in particular it singles out when neutrinos may be responsible for powering GRBs. The third work studies the tidal stripping of a white dwarf spiraling into a massive black hole. The stripped matter accretes onto the black hole producing a transient emission, presumably periodic, observable with X-ray and optical telescopes. At the same time the white dwarf emits gravitational waves as it spirals into the black hole.Astrophysicsilz2101PhysicsDissertationsInvestigations of the Band Structure and Morphology of Nanostructured Surfaces
http://academiccommons.columbia.edu/catalog/ac:135348
Knox, Kevin R.http://hdl.handle.net/10022/AC:P:10649Wed, 06 Jul 2011 00:00:00 +0000Two-dimensional electronic systems have long attracted interest in the physics and material science communities due to the exotic physics that arises from low-dimensional confinement. Studying the electronic behavior of 2D systems can provide insight into a variety of phenomena that are important to condensed-matter physics, including epitaxial growth, two-dimensional electron scattering and many-body physics. Correlation effects are strongly influenced by dimensionality, which determines the many-body excitations available to a system. In this dissertation, I examine the electronic structure of two very dierent types of two-dimensional systems: valence band electrons in single layer graphene and electronic states created at the vacuum interface of single crystal copper surfaces.The characteristics of both electronic systems depend intimately on the morphology of the surfaces they inhabit. Thus, in addition to discussing the respective band structures of these systems, a significant portion of this dissertation will be devoted to measurements of the surface morphology of these systems. Free-standing exfoliated monolayer graphene is an ultra-thin flexible membrane and, as such, is known to exhibit large out-of-plane deformation due to substrate and adsorbate interaction as well as thermal vibrations and, possibly, intrinsic buckling. Such crystal deformation is known to limit mobility and increase local chemical reactivity. Additionally, deformations present a measurement challenge to researchers wishing to determine the band structure by angle-resolved photoemission since they limit electron coherence in such measurements. In this dissertation, I present low energy electron microscopy and microprobe diffraction measurements, which are used to image and characterize corrugation in SiO2-supported and suspended exfoliated graphene at nanometer length scales. Diffraction line-shape analysis reveals quantitative differences in surface roughness on length scales below 20 nm which depend on film thickness and interaction with the substrate. Corrugation decreases with increasing film thickness, reflecting the increased stiffness of multilayer films. Specifically, single-layer graphene shows a markedly larger short range roughness than multilayer graphene. Due to the absence of interactions with the substrate, suspended graphene displays a smoother morphology and texture than supported graphene. A specific feature of suspended single-layer films is the dependence of corrugation on both adsorbate load and temperature, which is manifested by variations in the diffraction lineshape. The effects of both intrinsic and extrinsic corrugation factors will be discussed. Through a carefully coordinated study I show how these surface morphology measurements can be combined with angle resolved photoemission measurements to understand the role of surface corrugation in the ARPES measurement process. The measurements described here rely on the development of an analytical formulation for relating the crystal corrugation to the photoemission linewidth. I present ARPES measurements that show that, despite signicant deviation from planarity of the crystal, the electronic structure of exfoliated suspended graphene is nearly that of ideal, undoped graphene; the Dirac point is measured to be within 25 meV of EF . Further, I show that suspended graphene behaves as a marginal Fermi-liquid, with a quasiparticle lifetime which scales as (E - EF)-1; comparison with other graphene and graphite data is discussed. Image and surface states formed at the vacuum interface of a single crystal provide another example of a two dimensional electronic system. As with graphene, the surface quality and morphology strongly inuence the physics in this 2D electronic system. However, in contrast to graphene, which must be treated as a flexible membrane with continuous height variation, roughness in clean single crystal surfaces arises from lattice dislocations, which introduce discrete height variations. Such height variations can be exploited to generate a self assembled nano-structured surface. In particular, by making a vicinal cut on a single crystal surface, a nanoscale step array can be formed. A model system for such nanoscale self assembly is Cu(111). Cu(775) is formed by making an 8.5° viscinal cut of Cu(111) along the [11 -2] axis. The electronic states formed on the surface of this system, with a nanoscale step array of 14 Å terraces, shows markedly different behavior those formed on Cu(111). In this dissertation, I show that the tunability of a femtosecond optical parametric oscillator, combined with its high-repetition rate and short pulse length, provides a powerful tool for resonant band mapping of the sp surface and image states on flat and vicinal Cu(111)- Cu (775) surfaces, over the photon energy range from 3.9 to 5 eV. Since the time scale for excitation of the metal image state from the Cu surface state is comparable with the electron-electron equilibration time scale, sharp features are measured due to resonant excitation in the photoelectron energy distribution curves. In addition, I explore the range of photon energies and optical intensities which may be used for this approach and show that, despite the relatively high pump intensity, the 250 kHz repetition rate of this laser ameliorates the space-charge broadening and electron-energy shifting even for photon energies close to the vacuum edge. The strong excitation conditions generated by a femtosecond laser pulse applied to a Cu surface also allow the excitation and observation of a recently measured bulk state. In this dissertation I show that angle-resolved, tunable, two-photon photoemission (2PPE) can be used to map a bulk unoccupied band, viz. the Cu sp band 0 to 1 eV below the vacuum level, in the vicinity of the L point. This short-lived bulk band can be accessed using our setup due to the strong optical pump rate. I describe how photoemission from this state can be distinguished from photoemission from 2D states which is also present in the data. In particular, the variation of the initial-state energy with photon energy has a measured slope of ~ 1.64 in contrast with values of 1 or 2 observed for 2PPE from two-dimensional (2D) states. This unique variation illustrates the significant role of the perpendicular momentum of initial and final states in interpreting 2PPE data.Condensed matter physicskrk19PhysicsDissertationsProbing the Properties of the Molecular Adlayers on Metal Substrates: Scanning Tunneling Microscopy Study of Amine Adsorption on Au(111) and Graphene Nanoislands on Co(0001)
http://academiccommons.columbia.edu/catalog/ac:132245
Zhou, Huihttp://hdl.handle.net/10022/AC:P:10385Mon, 16 May 2011 00:00:00 +0000In this thesis, we present our findings on two major topics, both of which are studies of molecules on metal surfaces by scanning tunneling microscopy (STM). The first topic is on adsorption of a model amine compound, 1,4-benzenediamine (BDA), on the reconstructed Au(111) surface, chosen for its potential application as a molecular electronic device. The molecules were deposited in the gas phase onto the substrate in the vacuum chamber. Five different patterns of BDA molecules on the surface at different coverages, and the preferred adsorption sites of BDA molecules on reconstructed Au(111) surface, were observed. In addition, BDA molecules were susceptible to tip-induced movement, suggesting that BDA molecules on metal surfaces can be a potential candidate in STM molecular manipulations. We also studied graphene nanoislands on Co(0001) in the hope of understanding interaction of expitaxially grown graphene and metal substrates. This topic can shed a light on the potential application of graphene as an electronic device, especially in spintronics. The graphene nanoislands were formed by annealing contorted hexabenzocoronene (HBC) on the Co(0001) surface. In our experiments, we have determined atop registry of graphene atoms with respect to the underlying Co surface. We also investigated the low-energy electronic structures of graphene nanoislands by scanning tunneling spectroscopy. The result was compared with a first-principle calculation using density functional theory (DFT) which suggested strong coupling between graphene pi-bands and cobalt d-electrons. We also observed that the islands exhibit zigzag edges, which exhibits unique electronic structures compared with the center areas of the islands.Nanoscience, Molecular physics, Materials sciencehz2106Physics, Electrical EngineeringDissertationsDelta I Equals Three Halfs Kaon To Two Pion Decays Using Lattice Quantum Chromodynamics with Domain Wall Fermions
http://academiccommons.columbia.edu/catalog/ac:131995
Lightman, Matthewhttp://hdl.handle.net/10022/AC:P:10324Wed, 11 May 2011 00:00:00 +0000We calculate matrix elements for kaon to two pion decays in the Delta I = 3/2 channel using lattice gauge theory simulations. From these we can extract the decay amplitude A2, for which the real part is related to the decay rate and can be compared to the experimental result Re(A2) = 1.484 x 10^(-8) GeV, and for which the imaginary part is related to direct charge-parity violation in the neutral kaon system. We report the results of one simulation with nearly physical particle masses and kinematics, specifically mK = 509.0(9.1) MeV, mPi = 142.8(2.5) MeV, and EPiPi = 485.7(8.0) MeV. This simulation was performed on RBC/UKQCD 32^3 x 64, Ls = 32 lattices, using 2+1 dynamical flavors of domain wall fermions and a Dislocation Suppressing Determinant Ratio plus Iwasaki gauge action, and with an inverse lattice spacing a^(-1) = 1.373(24) GeV so that the spatial extent of the lattice is 4.60 fm and mPi*L = 3.3. We find that Re(A2) = 1.461(87)stat(200)sys x 10^(-8) GeV, in good agreement with the experimental value. We also find Im(A2) = -8.67(45)stat(1.95)sys x 10^(-13) GeV, and Im(A2)/Re(A2) = -5.93(27)stat(1.42)sys x 10^(-5), however the value of Im(A2) depends on a rough hypothesis for some of the renormalization constants which have not yet been calculated, and thus we quote a large systematic error. We also report the results of a simulation involving a variety of kaon and pion masses and momenta, which was conducted in order to study the dependence of the decay amplitude on particle masses and kinematics, and to study the effect of not having exactly physical masses and kinematics in the first simulation. The use of the quenched approximation and smaller spatial volume in this second simulation allowed for multiple masses to be simulated in a reasonable amount of time, but introduced an uncontrolled approximation and forced us to use pion masses a bit larger than the phys- ical mass. The study was conducted on 24^3 x 64, Ls = 16 lattices, with the quenched Doubly Blocked Wilson 2 gauge action, and an inverse lattice spacing of a^(-1) = 1.31(2) GeV. We find that an extrapolation to physical masses and kinematics yields values Re(A2) = 2.25(18)stat x 10^(-8) GeV and Im(A2) = -13.44(84)stat x 10^(-13) GeV. These results are significantly larger than those of the full dynamical simulation and of experiment. We attribute this mainly to the an inaccurate determination of the lattice spacing a using the rho mass, since it comes in as a^(-3) in the calculation of A2. Finally, a third simulation is performed with 2+1 dynamical flavors of domain wall fermions on a finer 32^3 x 64, Ls = 16 lattice, but only with pions that have nearly zero momentum. It, and the quenched simulation, are used mainly to estimate the systematic error in the first simulation, which is taken as the final result.Particle physicsmbl2108PhysicsDissertationsElectron-Muon Correlations in Proton+Proton and Deuteron+Gold Collisions at PHENIX
http://academiccommons.columbia.edu/catalog/ac:132092
Engelmore, Tatiahttp://hdl.handle.net/10022/AC:P:10354Wed, 11 May 2011 00:00:00 +0000This dissertation presents the first measurement of electron-muon azimuthal correlations at the PHENIX experiment at RHIC in 200 GeV proton-proton and deuteron-gold collisions. Electron-muon pairs result from the semileptonic decay of D mesons, which come from correlated charm pairs. The pairs are measured at forward rapidity, with η < 0.5 for the electron and 1.4 < η < 2.1 for the muon. Electron-muon pairs exhibit a characteristic peak at Δφ = π in the azimuthal distribution due to momentum conservation in the c\bar{c} decay, and this enables clear identification. The shape of the azimuthal pair distribution in p+p collisions helps us determine which hard scattering processes contribute to charm production, and it allows us to test NLO QCD predictions. The p+p result also serves as a baseline measurement for understanding heavy ion collisions. Pairs were also measured in d+Au collisions at forward rapidity in the deuteron-going direction, which is a kinematic region at which we expect suppression effects to be evident. The pair yield in d+Au was found to be suppressed relative to that in p+p. Also the peak in Δφ almost disappears in d+Au, indicating either a change in charm production mechanisms or interactions with the nuclear matter.Nuclear physicstke2101Physics, Nevis LaboratoriesDissertations