Identified charged particle azimuthal anisotropy in PHENIX at RHIC

Event anisotropy is expected to have sensitivity to the early stage of ultra-relativistic heavy-ion collisions at RHIC. The possible formation of a quark gluon plasma (QGP) could affect how the initial anisotropy in the space coordinate is transferred into the momentum space for the final state. The anisotropy parameter (v2) is an amplitude of the 2nd harmonic parameter of the azimuthal distribution with respect to the reaction plane. We present here v2 of identified and inclusive charged particles measured in the PHENIX central arm detector (|η| < 0.35) with respect to the reaction plane defined at |η| = 3 ∼ 4 in 200 GeV Au+Au collisions. We find that v2 increases from central to midcentral collisions reaching a maximum at about 50% of the geometric cross section and then decreases again for more peripheral collisions. As a function of transverse momentum in minimum-bias collisions, the v2 parameter increases linearly with pT up to pT ≃ 2 GeV/c and then saturates for inclusive charged particles. The v2 parameter of identified particles (π, π−, K, K−, p and p) follow a hydro-dynamic behavior up to 2 GeV/c in pT, where the lighter mass particles have larger v2 at a given pT. However there is an indication that this trend is reversed at around pT ≃ 2 GeV/c, where p and p have larger v2 than π and K.