Search for flavour-changing neutral-current couplings between the top-quark and the photon with the ATLAS detector at $\sqrt{s} = 13$ TeV

This note documents the search for flavour-changing neutral currents (FCNCs), which are strongly suppressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in $pp$ collisions at $\sqrt{s} = 13$ TeV during the LHC Run 2 corresponding to an integrated luminosity of 139 fb$^{-1}$. 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 as either one of the two signal categories, FCNC production or decay, or as background. No significant excess of events over the background prediction is observed and 95 % CL upper limits on the strength of left- and right-handed FCNC interactions are set. The 95 % CL bounds on the branching fractions (BRs) for the FCNC top-quark decays, estimated from both top-quark production and decay, are BR$(t\rightarrow u\gamma) < 0.85 \times 10^{-5}$ and BR$(t\to c\gamma) < 4.2 \times 10^{-5}$ for a left-handed $tq\gamma$ coupling, and BR$(t\to u\gamma) < 1.2 \times 10^{-5}$ and BR$(t\to c\gamma) < 4.5 \times 10^{-5}$ for a right-handed coupling.