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

摘要

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?1of proton–proton 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, ab-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 thetqγcoupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNCtγproduction via a left-handed (right-handed)tuγcoupling of 36 fb (78 fb) and on the branching ratio fort→γuof2.8×10?5(6.1×10?5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNCtγproduction via a left-handed (right-handed)tcγcoupling of 40 fb (33 fb) and on the branching ratio fort→γcof22×10?5(18×10?5).