Measurements of inclusive and differential cross-sections of tty production in the e+mu final state at 13 TeV with the ATLAS detector

The production of a top-antitop pair in association with a photon (tty) gives access to the electromagnetic top-photon coupling and, thus, a tty cross section measurement enables probing the coupling strength and structure. Deviations of the cross section would be an indication of physics beyond the Standard Model (BSM). While the process was first observed by ATLAS at the LHC at 7 TeV, the full Run 2 data collected with the ATLAS experiment now enables precise cross-section measurements. The e+mu channel is of particular interest due to high signal purity and its precise theory predictions. Inclusive and differential cross-section measurements are presented using proton-proton collision data corresponding to an integrated luminosity of 139/fb, collected with the ATLAS detector between 2015 and 2018 at 13 TeV centre-of-mass energy. Events with exactly one photon, one electron and one muon of opposite sign, and at least two jets, one of which must be b-tagged, are selected. Differential cross-sections as a function of several observables are unfolded to parton level and compared to state-of-the-art Monte Carlo simulations and next-to-leading order calculations.