Photon-Hadron Jet Correlations in p+p and Au+Au Collisions at sqrt(sₙN) = 200 GeV

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We report the observation at the Relativistic Heavy Ion Collider (RHIC) 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 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 (pQCD), 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 +/- 0.12(stat) +/- 0.09(syst) for hadrons of 3 < pₜ < 5 in coincidence with photons of 5 < pₜ < 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 = pₜʰadron/pₜᵖhoton, as expected for a measure 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.