Azimuthal anisotropy ($v_2$) of high-p${}_T$ ${\pi }^0$ and direct $\gamma$ in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

Abstract

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.

arXiv comment4 pages, 2 figures, Hot Quarks 2010, LaLonde France

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• High-$p_{{}_T}$ Direct Photon Azimuthal Correlation Measurements