Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb 121 of pp collisions at 1as = 13 TeV with the ATLAS experiment

A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb 121 of proton\u2013proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tq\u3b3 coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC t\u3b3 production via a left-handed (right-handed) tu\u3b3 coupling of 36 fb (78 fb) and on the branching ratio for of (). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC t\u3b3 production via a left-handed (right-handed) tc\u3b3 coupling of 40 fb (33 fb) and on the branching ratio for of ()