Azimuthal anisotropy (v2) of high-pT π0 and direct γ in Au+Au collisions at = 200 GeV

Abstract

Preliminary results from the STAR collaboration of the azimuthal anisotropy (v2) of π0 and direct photon (γdir) at high transverse momentum (pT) from Au+Au collisions at center-of-mass energy = 200 GeV are presented. A shower-shape analysis is used to select a sample free of direct photons (π0) and a sample rich in direct photons γrich. The relative contribution of background in the γrich sample is determined assuming no associated charged particles nearby γdir. The v2 of direct photons (vγdir2) at mid-rapidity (|ηγdir| < 1) and high pT (8 < pγdirT < 16 GeV/c) is extracted from those of π0 and neutral particles measured in the same kinematic range. In mid-central Au+Au collisions (10–40%), the v2 of π0 (vπ02(pT)) and charged particles (vch2(pT)) are found to be ~ 0.12 and nearly independent of pT. The measured vγdir2(pT) is positive finite and systematically smaller than that of π0 and charged particles by a factor of ~ 3. Although the large vπ02 at such high pT might be partially due to the path-length dependence of energy loss, the non-zero value of vγdir2 indicates a bias of the reaction plane determination due to the presence of jets in the events. Systematic studies are currently in progress.