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

TL;DR

This study measures the azimuthal anisotropy ($v_{2}$) of high-$p_{T}$ $b^{0}$ and direct photons in Au+Au collisions at 200 GeV, revealing a smaller $v_{2}$ for direct photons compared to $b^{0}$ and charged particles, with implications for jet bias and energy loss.

Contribution

First measurement of $v_{2}$ for high-$p_{T}$ direct photons in Au+Au collisions at 200 GeV, highlighting the bias in reaction plane determination due to jets.

Findings

$v_{2}^{b^{0}}$ and charged particles are about 0.12 and nearly constant with $p_{T}$.

$v_{2}^{b^{0}}$ is significantly larger than $v_{2}^{ ext{direct }b}$, which is positive but smaller.

Presence of non-zero $v_{2}^{ ext{direct }b}$ suggests jet bias in reaction plane measurement.

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.