Photon-hadron jet correlations in p+p and Au+Au collisions at sNN=200 GeV

Abstract

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, IAA, in central Au+Au collisions, is 0.32±0.12stat±0.09syst for hadrons of 3<pTh<5 in coincidence with photons of 5<pTγ<15 GeV/c. The suppression is comparable to that observed for high-pT single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, zT pTh/pTγ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss. © 2009 The American Physical Society.