Theses Doctoral

Open Heavy Flavor Production in Relativistic Heavy Ion Collisions at LHC

Tian, Yun

ATLAS measurements of the production of muons from heavy flavor decays in √sNN = 2.76 TeV Pb+Pb collisions and √s = 2.76 TeV pp collisions at the LHC are presented. Integrated luminosities of 0.14 nb−1 and 570 nb−1 are used for the Pb+Pb and pp measurements, respectively. The measurements are performed over the transverse momentum range 4 < pT < 14 GeV and for five Pb+Pb centrality intervals. Backgrounds arising from in-flight pion and kaon decays, hadronic showers, and mis-reconstructed muons are statistically re- moved using a template fitting procedure. The heavy flavor muon differential cross-sections and per-event yields are measured in pp and Pb+Pb collisions, respectively. The nuclear modification factor, RAA, obtained from these is observed to be independent of pT, within uncertainties, and to be less than unity, which indicates suppressed production of heavy flavor muons in Pb+Pb collisions. For the 0–10% most central Pb+Pb events, the measured RAA is ∼ 0.35. The azimuthal modulation of the heavy flavor muon yields is also measured and the associated Fourier coefficients vn for n=2, 3 and 4 are given as a function of pT and centrality. They vary slowly with pT and show a systematic variation with centrality that is characteristic of other anisotropy measurements. The measured RAA and vn values are also compared with theoretical calculations.
We also present the ATLAS di-muon azimuthal correlation from heavy flavor decay in √√sNN = 5.02 TeV Pb+Pb collisions and s = 5.02 TeV pp collisions. Heavy flavor muons with 4 < pT < 8 GeV are selected. The azimuthal correlations are measured for both same sign muon pairs and opposite sign muon pairs. Comparing with the azimuthal correlations in pp data, Pb+Pb results are observed to have more broadening. Central collisions are also more broadened than peripheral collisions. The integrated luminosities used in the dimuon measurement are 26 pb−1 for the pp data and 0.49 nb−1 for the Pb+Pb data. In ultra-relativistic heavy-ion collisions, dilepton pairs may be produced through the interaction of the large electromagnetic fields of the nuclei. A measurement of γγ → μ+μ− in inclusive Pb+Pb collisions at √sNN = 5.02 TeV using 0.49 nb−1 is presented in this thesis. The contribution from background sources of dimuons is removed using a template fit method. The angular and transverse momentum correlations between the muons are measured as a function of collision centrality. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of dimuon production in ultra-peripheral collisions. The correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with attenuation of the muons while passing through the hot matter produced in the collision.


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More About This Work

Academic Units
Thesis Advisors
Cole, Brian A.
Ph.D., Columbia University
Published Here
June 24, 2018