Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations

Abstract

Charged-particle spectra associated with direct photon (${\ensuremath{\gamma}}_{\mathrm{dir}}$) and ${\ensuremath{\pi}}^{0}$ are measured in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at center-of-mass energy $\sqrt{{s}_{{}_{\mathit{NN}}}}=200$ GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between ${\ensuremath{\gamma}}_{\mathrm{dir}}$ and ${\ensuremath{\pi}}^{0}$. Assuming no associated charged particles in the ${\ensuremath{\gamma}}_{\mathrm{dir}}$ direction (near side) and small contribution from fragmentation photons (${\ensuremath{\gamma}}_{\mathrm{frag}}$), the associated charged-particle yields opposite to ${\ensuremath{\gamma}}_{\mathrm{dir}}$ (away side) are extracted. In central $\mathrm{Au}+\mathrm{Au}$ collisions, the charged-particle yields at midrapidity ($|\ensuremath{\eta}|<1$) and high transverse momentum ($3<{p}_{T}^{\mathrm{assoc}}<16$ GeV/$c$) associated with ${\ensuremath{\gamma}}_{\mathrm{dir}}$ and ${\ensuremath{\pi}}^{0}$ ($|\ensuremath{\eta}|<0.9$, $8<{p}_{T}^{\mathrm{trig}}<16$ GeV/$c$) are suppressed by a factor of 3--5 compared with $p+p$ collisions. The observed suppression of the associated charged particles is similar for ${\ensuremath{\gamma}}_{\mathrm{dir}}$ and ${\ensuremath{\pi}}^{0}$ and independent of the ${\ensuremath{\gamma}}_{\mathrm{dir}}$ energy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge.