Azimuthal anisotropy of charged particles at high transverse momenta in PbPb collisions at sqrt(s[NN]) = 2.76 TeV

TL;DR

This paper measures the azimuthal anisotropy (v2) of charged particles in PbPb collisions at 2.76 TeV across a wide transverse momentum range, revealing how anisotropy varies from hydrodynamic flow to parton energy loss effects.

Contribution

It provides the first detailed measurement of v2 at high transverse momenta up to 60 GeV in PbPb collisions at the LHC, extending understanding of medium effects on particle emission.

Findings

v2 increases up to about 3 GeV and then decreases at higher pt

Anisotropy persists up to at least 40 GeV

Results suggest a transition from flow-dominated to energy-loss-dominated regimes

Abstract

The azimuthal anisotropy of charged particles in PbPb collisions at nucleon-nucleon center-of-mass energy of 2.76 TeV is measured with the CMS detector at the LHC over an extended transverse momentum (pt) range up to approximately 60 GeV. The data cover both the low-pt region associated with hydrodynamic flow phenomena and the high-pt region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter (v2) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0-60% most-central events, the observed v2 values are found to first increase with pt, reaching a maximum around pt = 3 GeV, and then to gradually decrease to almost zero, with the decline persisting up to at least pt = 40 GeV over the full centrality range measured.