One more ingredient for energy loss quantification

Abstract

The recent results at RHIC for direct $\gamma$-charged hadron azimuthal correlations in heavy-ion collisions are presented. We use these correlations to study the color charge density of the medium through the medium-induced modification of high-p${}_T$ parton fragmentation. Azimuthal correlations of direct photons at high transverse energy (8 $<$ E${}_T$ $<$ 16 GeV) with away-side charged hadrons of transverse momentum (3 $<$ p${}_T$ $<$ 6 GeV/c) have been measured over a broad range of centrality for $Au+Au$ collisions and $p+p$ collisions at $\sqrt{s_{NN}}$ = 200 GeV in the STAR experiment. A transverse shower shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the direct photons and photons from the decays of high-p${}_T$ ${\pi }^0$. The per-trigger away-side yield of direct $\gamma$ is smaller than from ${\pi }^0$ triggers in the same centrality class. Within the current uncertainty the recoil suppression in central $Au+Au$ collisions I${}_{CP}$ of direct $\gamma$ and ${\pi }^0$ are similar.

5 pages, 7 figures, Hot Quarks 2008