Studying Parton Energy Loss in Heavy-Ion Collisions via Direct-Photon and Charged-Particle Azimuthal Correlations

Charged-particle spectra associated with direct photon (${\gamma }_{dir}$) and ${\pi }^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{{}_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between ${\gamma }_{dir}$ and ${\pi }^0$. Assuming no associated charged particles in the ${\gamma }_{dir}$ direction (near side) and small contribution from fragmentation photons (${\gamma }_{frag}$), the associated charged-particle yields opposite to ${\gamma }_{dir}$ (away side) are extracted. At mid-rapidity ($\mathrm{\mid }\eta \mathrm{\mid }<0.9$) in central Au+Au collisions, charged-particle yields associated with ${\gamma }_{dir}$ and ${\pi }^0$ at high transverse momentum ($8<p_T^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $\mathrm{\mid }\eta \mathrm{\mid }<1$ and $3< p_{T}^{assoc} < 16$GeV\mathrm{/}$c$,issimilarfor$\gamma_{dir}$and$\pi^0$, and independent of the ${\gamma }_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.