Study of photon-jet momentum correlations in Pb+Pb and $pp$ collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with ATLAS

Measurements of the azimuthal opening angle and transverse momentum balance between isolated photons and inclusive jets are presented using 0.49 nb$^{-1}$ of Pb+Pb collision data at $\sqrt{s_\mathrm{NN}}=5.02$~TeV and 26 pb$^{-1}$ of $pp$ collision data at $\sqrt{s}=5.02$~TeV with the ATLAS detector at the LHC. Photons are required to have transverse momentum $60 < p_\mathrm{T}^{\gamma} < 200$~GeV~and pseudorapidity $\left|\eta^{\gamma}\right| < 2.37$ (excluding the region $1.37 < \left|\eta^{\gamma}\right| < 1.52$), and to satisfy identification and isolation requirements. Jets are clustered from energy deposits in the calorimeter using the anti-$k_{t}$ algorithm with parameter $R=0.4$, and are required to have $p_\mathrm{T} > 30$~\GeV\ and $\left|\eta\right| < 2.1$. Photons are paired inclusively with all jets in the event, and the transverse momentum balance of the pairs with azimuthal opening angle $\Delta\phi > 7\pi/8$ is studied. The jet-to-photon $p_\mathrm{T}$ ratio, $x_{\mathrm{J\gamma}}$, measured in data, is compared to that of simulated photon+jet events. In $pp$ collisions, the distributions are well described by Monte Carlo simulation. In Pb+Pb collisions, the $x_{\mathrm{J\gamma}}$ distribution is observed to be modified with respect to the simulation in a way that is systematic in centrality and $p_\mathrm{T}^{\gamma}$, in a manner consistent with the picture of parton-energy loss in the hot nuclear medium.