Azimuthal anisotropy ($v_{2}$) of high-p$_{T}$ $π^{0}$ and direct $γ$ in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

Abstract

Preliminary results from the STAR collaboration of the azimuthal anisotropy $(v_{2})$ of $π^{0}$ and direct photon ($γ_{dir}$) at high transverse momentum (p$_{T}$) from Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$~GeV are presented. A shower-shape analysis is used to select a sample free of direct photons ($π^0$) and a sample rich in direct photons $γ_{rich}$. The relative contribution of background in the $γ_{rich}$ sample is determined assuming no associated charged particles nearby $γ_{dir}$. The $v_{2}$ of direct photons ($v_{2}^{γ_{dir}}$) at mid-rapidity ($|η^{γ_{dir}}|<1$) and high p$_{T}$ ($8< p_{T}^{γ_{dir}}<16$~GeV/$c$) is extracted from those of $π^{0}$ and neutral particles measured in the same kinematic range. In mid-central Au+Au collisions (10-40$\%$), the $v_{2}$ of $π^0$ ($v_{2}^{π^{0}}(p_{T})$) and charged particles ($v_{2}^{ch}(p_{T})$) are found to be $\sim$ 0.12 and nearly independent of p$_{T}$. The measured $v_{2}^{γ_{dir}}(p_{T})$ is positive finite and systematically smaller than that of $π^{0}$ and charged particles by a factor of $\sim$ 3. Although the large $v_{2}^{π^{0}}$ at such high p$_{T}$ might be partially due to the path-length dependence of energy loss, the non-zero value of $v_{2}^{γ_{dir}}$ indicates a bias of the reaction plane determination due to the presence of jets in the events. Systematic studies are currently in progress.