Observation of Single-Top-Quark Production in Association with a Photon Using the ATLAS Detector Aad, G. Aguilar Saavedra, Juan Antonio Rodríguez Chala, Mikael Atlas Collaboration 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. 2023-12-05T11:00:29Z 2023-12-05T11:00:29Z 2023-10-30 journal article G. Aad et al. (ATLAS Collaboration) Phys. Rev. Lett. 131, 181901 [https://doi.org/10.1103/PhysRevLett.131.181901] https://hdl.handle.net/10481/86034 10.1103/PhysRevLett.131.181901 eng http://creativecommons.org/licenses/by/4.0/ open access Atribución 4.0 Internacional American Physical Society