High-$p_{_{T}}$ Direct Photon Azimuthal Correlation Measurements

TL;DR

This study measures azimuthal correlations of high-$p_{T}$ direct photons with charged hadrons in Au+Au collisions, revealing smaller elliptic flow and greater suppression of recoiled hadrons compared to neutral pions, providing insights into quark-gluon plasma properties.

Contribution

It presents the first detailed comparison of direct photon and neutral pion azimuthal correlations at high $p_{T}$ in heavy-ion collisions, highlighting differences in flow and suppression patterns.

Findings

Direct photon $v_2$ is consistent with zero at high $p_T$.

Recoiled charged hadron spectra show more suppression for direct photon triggers.

Differences in suppression suggest medium effects on parton energy loss.

Abstract

The azimuthal correlations of direct photons ($\gamma_{_{dir}}$) with high transverse momentum ($p_{_{T}}$), produced at mid-rapidity ($|\eta^{\gamma_{_{dir}}}|<1$) in Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$~GeV, are measured and compared to those of neutral pions ($\pi^{0}$) in the same kinematic range. The measured azimuthal elliptic anisotropy of direct photon, $v_{_{2}}^{\gamma_{_{dir}}}(p_{_{T}})$, 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 ($2.4 <|\eta|< 4.0$) in reconstructing the event plane. The associated charged hadron spectra recoiled from $\gamma_{_{dir}}$ show more suppression than those recoiled from $\pi^{0}$ $(I_{AA}^{\gamma_{_{dir}}-h^{\pm}} < I_{AA}^{\pi^{0}-h^{\pm}})$ in the new measured kinematic range $12< p_{_{T}}^{\gamma_{_{dir}},\pi^{0}}<24$~GeV/$c$ and $3< p_{_{T}}^{assoc}<24$~GeV/$c$.