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