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  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⁢(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.