Abstract
The Fourier coefficients v2 and v3 characterizing the anisotropy of the azimuthal distribution of charged particles produced in PbPb collisions at sNN=5.02 TeV are measured with data collected by the CMS experiment. The measurements cover a broad transverse momentum range, 1<pT<100 GeV/c. The analysis focuses on the pT>10 GeV/c range, where anisotropic azimuthal distributions should reflect the path-length dependence of parton energy loss in the created medium. Results are presented in several bins of PbPb collision centrality, spanning the 60% most central events. The v2 coefficient is measured with the scalar product and the multiparticle cumulant methods, which have different sensitivities to initial-state fluctuations. The values from both methods remain positive up to pT∼60–80 GeV/c, in all examined centrality classes. The v3 coefficient, only measured with the scalar product method, tends to zero for pT≳20 GeV/c. Comparisons between theoretical calculations and data provide new constraints on the path-length dependence of parton energy loss in heavy ion collisions and highlight the importance of the initial-state fluctuations.
Highlights
Introduction tohfemSLeaHvsesCraeilnnoebPrgsbeyPrbvpaectroiollnnisusicomlneasodnaetpaat√iRrsHN√NICs=NiNn2=.A7u62A0ua0ncdoGlel5iVs.0io2[n1sT–e4aV]t centerand at [5,6,7,8,9,10]establish that high-energy partons lose a significant fraction of their energy while traversing the hot and dense medium created in these collisions
The analysis focuses on the pT > 10 GeV/c range, where anisotropic azimuthal distributions should reflect the path-length dependence of parton energy loss in the created medium
The v2 and v3 coefficients are determined, as a function of collision centrality, over the widest transverse momentum range studied to date
Summary
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter providing a 3.8 T field. Within the solenoid volume there are a silicon pixel and strip tracker detector, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. The silicon tracker measures charged particles within |η| < 2.5 and provides a pT resolution of about 1.5%. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. The detailed Monte Carlo simulation of the CMS detector response is based on Geant4 [33]
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