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

The ATLAS collaboration
Aad, Georges , Abbott, Braden Keim , Abbott, D.C. , Abeling, Kira , Abidi, Haider , Aboulhorma, Asmaa , Abramowicz, Halina , Abreu, Henso , Abulaiti, Yiming , Abusleme Hoffman, A.C.
Phys.Rev.Lett. 131 (2023) 181901, 2023.

Abstract (data abstract)
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 $688\,\pm\,23 \, \mathrm{(stat.)} \, ^{+75}_{-71} \, \mathrm{(syst.)}\,$fb, to be compared with the Standard Model prediction of $515\,^{+36}_{-42}\,$fb at next-to-leading order in QCD.

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