Abstract
The anisotropy of the azimuthal distributions of charged particles produced in PbPb collisions with a nucleon-nucleon center-of-mass energy of 2.76 TeV is studied with the CMS experiment at the LHC. The elliptic anisotropy parameter defined as the second coefficient in a Fourier expansion of the particle invariant yields, is extracted using the event-plane method, two- and four-particle cumulants, and Lee--Yang zeros. The anisotropy is presented as a function of transverse momentum (pt), pseudorapidity (eta) over a broad kinematic range: 0.3 < pt < 20 GeV, abs(eta) < 2.4, and in 12 classes of collision centrality from 0 to 80%. The results are compared to those obtained at lower center-of-mass energies, and various scaling behaviors are examined. When scaled by the geometric eccentricity of the collision zone, the elliptic anisotropy is found to obey a universal scaling with the transverse particle density for different collision systems and center-of-mass energies.
Highlights
The azimuthal anisotropy of emitted charged particles is an important feature of the hot, dense medium produced in heavy-ion collisions and has contributed to the suggestion of a strongly coupled quark-gluon plasma being produced in nucleus-nucleus collisions at RHIC [1,2,3,4,5]
The beam scintillation counters (BSCs) are a series of scintillator tiles which are sensitive to almost the full PHOBOS(AuAu) CERES (PbAu) NA49 (PbPb) interaction cross section
For each group of events that comprises a centrality class, we evaluate a set of quantities that characterize the initial geometry of the collisions using a Monte Carlo (MC) Glauber model
Summary
The azimuthal anisotropy of emitted charged particles is an important feature of the hot, dense medium produced in heavy-ion collisions and has contributed to the suggestion of a strongly coupled quark-gluon plasma (sQGP) being produced in nucleus-nucleus collisions at RHIC [1,2,3,4,5]. If the nucleon density within the nuclei is continuous, the initial nuclear overlap region is spatially asymmetric with an “almondlike” shape. The acceleration is greatest in the direction of the largest pressure gradient, that is, along the short axis of the almond. This results in an anisotropic azimuthal distribution of the final-state hadrons.
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