We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in $\mathrm{Au}+\mathrm{Au}$ 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 ($\gamma \text{−}h$) 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, $I_{\mathrm{AA}}$, in central $\mathrm{Au}+\mathrm{Au}$ collisions, is $0.32\pm 0.{12}^{\mathrm{stat}}\pm 0.{09}^{\mathrm{syst}}$ for hadrons of $3<p_T^h<5$ in coincidence with photons of $5<p_T^{\gamma }<15$ GeV/$c$. The suppression is comparable to that observed for high-$p_T$ single hadrons and dihadrons. The direct photon associated yields in $p+p$ collisions scale approximately with the momentum balance, $z_T\equiv p_T^h/p_T^{\gamma }$, as expected for a measurement of the away-side parton fragmentation function. We compare to $\mathrm{Au}+\mathrm{Au}$ collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

• Received 20 March 2009

DOI:https://doi.org/10.1103/PhysRevC.80.024908