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

This note reports the observation of single top quarks produced together with a photon using 139 fb$^{−1}$ of data collected with the ATLAS detector in 13 TeV proton–proton collisions 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 acceptance, the fiducial cross section is measured to be 580 ± 19 (stat.) ± 63 (syst.) fb, compared to the Standard Model prediction of 406 $^{+25}_{-32}$ fb.