Differential cross-section measurements of the associated production of top quarks with a photon at 13 TeV with the ATLAS detector

Absolute and normalised differential cross-sections for the associated production of top quarks with a photon are measured in proton-proton collisions data at a centre-of-mass energy of 13 TeV. The data were collected by the ATLAS detector during the full Run 2 of the LHC with an integrated luminosity of 139 fb^{-1}. The measurements are performed in the electron-muon decay channel in a fiducial phase space at parton level. The signal region is characterised by events with exactly one hard photon, one electron and one muon of opposite charge, at least two jets, among which at least one is b-tagged, and missing transverse energy. The differential cross-sections are measured as functions of photon kinematic observables, the minimum angular distance between the photon and the leptons, and angular separations between the two leptons. The results are compared to the most recent theory prediction at next-to-leading-order accuracy in QCD and state-of-the-art Monte Carlo simulations. The measurements are found to be in agreement with the Standard Model predictions within uncertainties.