Measurement of photon-jet $p_\mathrm{T}$ correlations in 5.02 TeV Pb+Pb and $pp$ collisions with ATLAS

Measurements of the 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 25 pb$^{-1}$ of $pp$ collision data at $\sqrt{s}=5.02$ TeV recorded with the ATLAS detector at the LHC. Photons with $p_\mathrm{T}^{\gamma} > 63.1$ GeV and $\left|\eta^{\gamma}\right| < 2.37$ are paired inclusively with all jets that have $p_\mathrm{T} > 31.6$~GeV and $\left|\eta\right| < 2.8$ in the event. The transverse momentum balance given by the jet-to-photon $p_\mathrm{T}$ ratio, $x_\mathrm{J\gamma}$, are measured for pairs with azimuthal opening angle $\Delta\phi > 7\pi/8$. Distributions of the per-photon jet yield $(1/N^\gamma)(\mathrm{d}N/\mathrm{d}x_\mathrm{J\gamma})$ are corrected via a two-dimensional unfolding procedure for detector effects and reported at the particle level for the first time. In $pp$ collisions, the distributions are well described by Monte Carlo event generators. In Pb+Pb collisions, the $x_\mathrm{J\gamma}$ distribution is modified with increasing centrality, consistent with the picture of parton-energy loss in the hot nuclear medium. The data are compared to a suite of energy loss models and calculations.