Photon and photon+jet probes of small and large collision systems with ATLAS

Measurements of photon and photon+jet production can shed light into the initial- and final-state interactions in collisions involving large nuclei. The ATLAS detector at the Large Hadron Collider measures prompt isolated photons over a broad kinematic range with barrel and endcap liquid argon electromagnetic calorimeters. In proton+proton collisions, such measurements test perturbative QCD and Monte Carlo descriptions of these processes. In proton+lead collisions, these measurements constrain the degree to which parton distribution functions are modified in nuclei and partons lose energy in the cold nuclear environment. Finally, in lead+lead collisions, where a large, long-lived QGP region is created, measurements of photon-tagged jet production and modification can probe how partons lose energy through interactions with the QGP. In this talk, we describe the latest ATLAS results on photon and photon+jet production in a wide variety of collision systems, including prospects with the high-luminosity lead+lead data recorded in 2018.