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Academic Commons Search Resultsen-usK_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 physicsada2129PhysicsDissertationsSearch 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 physicsnn2221PhysicsDissertationsSearch 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 physicsft2181DissertationsA 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 physicsdhu2101PhysicsDissertationsSearching 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 physicsajf2140PhysicsDissertationsSearch 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, PhysicsDissertationsDelta 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 physicsmbl2108PhysicsDissertations