Measurements of inclusive and differential cross-sections of $t\bar{t}\gamma$ production in the $e\mkern-2mu\mu$ channel at 13 TeV with the ATLAS detector

Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 139 fb$^{-1}$. The data were collected by the ATLAS detector at the LHC during Run 2 between 2015 and 2018 at a centre-of-mass energy of 13 TeV. The measurements are performed in the top-quark pair $e\mkern-2mu\mu$ final state in a fiducial volume defined at parton level. Events with exactly one photon, one electron and one muon of opposite sign, and at least two jets, out of which at least one is $b$-tagged, are selected. The fiducial cross-section is measured to be $44.2\pm2.6$ fb. Differential cross-sections as functions of several observables are compared to state-of-the-art Monte Carlo simulations and next-to-leading order theoretical calculations. These include cross-sections as functions of the photon transverse momentum and absolute pseudorapidity and angular variables related to the photon and the leptons and between the two leptons in the event. All measurements are in agreement with the predictions.