Prompt photon production in $\sqrt{s_\mathrm{NN}} = 8.16$ TeV $p$+Pb collisions with ATLAS

The inclusive production cross-section for isolated, prompt photons in $p$+Pb collisions at $\sqrt{s_\mathrm{NN}} = 8.16$~TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of $162$~nb$^{-1}$. The cross-section is measured as a function of photon transverse energy from $25$ to $500$~GeV and over nearly five units of pseudorapidity, including kinematic regions in both the downstream proton-- and nucleus--going directions. Results are reported in the nucleon--nucleon center of mass collision frame. The nuclear modification factor $R_{p\mathrm{Pb}}$ is reported using a extrapolation, derived using NLO pQCD, of a previous measurement of photon production in $pp$ collisions at $8$~TeV. The cross-sections and $R_{p\mathrm{Pb}}$ values are compared to the results of a next-to-leading order perturbative QCD calculation and to the expectations based on a picture of the energy loss of partons incoming to the hard scattering. At large nucleus-going rapidity, the cross-section is suppressed due to the different up and down valence quark content of the nucleus relative to that of the proton. The data are consistent with global analysis of the parton densities in nuclei, and disfavor a large amount of energy loss in the initial state.