Azimuthal elliptic anisotropy ($v_{_{2}}$) of high-$p_{_{T}}$ direct $\gamma$ in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

TL;DR

This study measures the azimuthal elliptic anisotropy of high-$p_{T}$ direct photons in Au+Au collisions at 200 GeV, finding it smaller than that of neutral pions and consistent with zero at high transverse momentum.

Contribution

First measurement of $v_{2}$ for high-$p_{T}$ direct photons in heavy-ion collisions at RHIC energies.

Findings

$v_{2}$ for direct photons is smaller than for $$ pions.

$v_{2}$ for direct photons is consistent with zero at high $p_{T}$.

Results suggest limited photon anisotropy at high transverse momentum.

Abstract

Preliminary results from the STAR collaboration for the azimuthal elliptic anisotropy $(v_{_{2}})$ of high transverse momentum ($p_{_{T}}$) direct photons ($\gamma_{_{dir}}$) produced at mid-rapidity ($|\eta^{\gamma_{_{dir}}}|<1$) in Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$~GeV are presented, and compared to the measured $(v_{_{2}})$ of neutral pions ($\pi^{0}$) in the same kinematic range. The electromagnetic transverse shower profile is used to distinguish $\pi^{0}$ from direct photons. The measured $v_{_{2}}^{\gamma_{_{dir}}}(p_{_{T}})$ at high $p_{_{T}}$ ($8< p_{_{T}}^{\gamma_{_{dir}}}<20$~GeV/$c$) is found to be smaller than that of $\pi^{0}$ and consistent with zero when using the forward detectors in determining the event plane.