PHENIX measurement of direct photon-triggered two-particle correlations in heavy ion collisions and its implication of the medium induced energy loss.

Abstract

Direct photon-hadron correlations are an excellent probe for QCD effects, including parton energy loss in the Quark-Gluon Plasma. At leading order, direct photons balance the pT of the away-side jet. In addition, as a colorless probe, direct photons do not interact strongly with the colored medium providing a less biased trigger than a single high-pT hadron. PHENIX has measured direct photon-triggered two-particle azimuthal correlations in a variety of collision systems at \sqrt{s_NN} = 200 GeV. In d+Au collisions, no modification of the per-trigger pair yields compared to p+p collisions was observed constraining the amount of cold nuclear matter effects in such collisions. In A+A collisions, direct photons have been identified statistically, as well as using an isolation cut. Combining data sets from different collisions systems allows us to quantify the transition from suppression at high zT ( = p_T^{hadron}/p_T^{\gamma}) to the enhancement of low zT particles relative to p+p, and to study this transition as a function of trigger pT. Integrating per-trigger yields in different ranges of the away-side gives insights on the redistribution of energy within the jet. The implication for our understanding of energy loss mechanisms will be discussed.