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

Citation STAR Collaboration et al. " Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations. Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Charged-particle spectra associated with direct photon (γ dir) and π 0 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 γ dir and π 0. Assuming no associated charged particles in the γ dir direction (near side) and small contribution from fragmentation photons (γ frag), the associated charged-particle yields opposite to γ dir (away side) are extracted. In central Au + Au collisions, the charged-particle yields at midrapidity (|η| < 1) and high transverse momentum (3 < p assoc T < 16 GeV/c) associated with γ dir and π 0 (|η| < 0.9, 8 < p trig T < 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 γ dir and π 0 and independent of the γ 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.