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

Charged-particle spectra associated with direct photon (γdir) and π⁰ are measured in $p+p$ and Au+Au collisions at center-of-mass energy √s_NN=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between γdir and π⁰. Assuming no associated charged particles in the γdir direction (near side) and small contribution from fragmentation photons (γfrag), the associated charged-particle yields opposite to γdir (away side) are extracted. In central Au+Au collisions, the charged-particle yields at midrapidity (|η|<1) and high transverse momentum (3<pTᵃˢˢᵒᶜ<16 GeV/c) associated with γdir and π⁰ (|η|<0.9, 8<pTᵗʳⁱᵍ<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 γdir and π⁰ and independent of the γdir 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.