Observation of single-top-quark production in association with a photon using the ATLAS detector

This Letter reports the observation of single top quarks produced together with a photon using 139 fb$^{-1}$ of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. The analysis uses the presence of a forward jet, characteristic of $t$-channel production, and separates the signal from the background with neural networks. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be 688 $\pm$ 23 (stat.) $^{+75}_{-71}$ (syst.) fb, to be compared with the Standard Model prediction of 515 $^{+36}_{-42}$ fb at next-to-leading order in QCD.