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, which directly probes the electroweak coupling of the top quark. The analysis uses $139\text{}\text{}{\mathrm{fb}}^{-1}$ of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be $688±23\left(\mathrm{stat}\right){\text{}}_{-71}^{+75}\left(\mathrm{syst}\right)\text{}\text{}\mathrm{fb}$, to be compared with the standard model prediction of ${515}_{-42}^{+36}\text{}\text{}\mathrm{fb}$ at next-to-leading order in QCD.