Cross-section measurement of tt̄γ production in pp collisions at √s = 8 and 13 TeV with the ATLAS experiment

The cross-section measurement of top-quark pair production in association with a photon (tt̄γ) probes the electromagnetic coupling of the top quark. The inclusive and differential fiducial cross sections of tt̄γ are measured with the data collected by the ATLAS detector from proton-proton collisions at the Large Hadron Collider, at the centre-of-mass energy of √s = 8 TeV, with a corresponding integrated luminosity of 20.2 fb−1, and at the centre-of-mass energy of √s = 13 TeV, with a corresponding integrated luminosity of 36.1 fb−1. The measurements are performed in the single-lepton final state. Signal events in the √s = 8 (13) TeV measurement are selected by requiring one photon with a transverse momentum of pT > 15 (20) GeV, one isolated electron or muon, and at least four jets where at least one of them is originated from a b-quark. The fiducial region is defined to be close to the selection requirements. The inclusive cross-sections in √s = 8 and 13 TeV are measured to be 139 ± 7 (stat.) ± 17 (syst.) fb and 521 ± 9 (stat.) ± 41 (sys.) fb, respectively, in agreement with the corresponding next-to-leading-order theoretical predictions of 151 ± 24 fb and 495 ± 99 fb. The differential cross-sections are measured as a function of transverse momentum and absolute pseudorapidity of the photon, in both √s = 8 and 13 TeV data-sets, and as a function of the angular distance between the photon and the lepton in the √s = 13 TeV data-set. All differential measurements are in agreement with the theoretical predictions at next-to-leading-order accuracy.