Search for flavour-changing neutral-current couplings between the top quark and the photon with the ATLAS detector at √s=13TeV

Abstract

This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly sup-pressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in ppcollisions at √s=13TeV during Run2 of the LHC, corresponding to an integrated luminosity of 139fb−1. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant ex-cess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left-and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t→uγ) <0.85 (0.88+0.37−0.25) ×10−5and B(t→cγ) <4.2 (3.40+1.35−0.95) ×10−5for a left-handed tqγcou-pling, and B(t→uγ) <1.2 (1.20+0.50−0.33) ×10−5and B(t→cγ) <4.5 (3.70+1.47−1.03) ×10−5for a right-handed coupling.