Probing the top-quark coupling to the photon through the cross-section measurement of $t\bar{t}\gamma$ production in $\textit{pp}$ collisions with the ATLAS detector

The cross section of top-quark pair production in association with a photon is measured in proton–proton collisions at a centre-of-mass energy $\sqrt{s}$ = 8 TeV with 20.2 fb$^{-1}$of data and at $\sqrt{s}$ = 13 TeV with 36.1 fb$^{-1}$ of data. Both datasets are collected by the ATLAS detector at the Large Hadron Collider. The measurements provide a direct probe of the electromagnetic top-quark coupling. The $t\bar{t}\gamma$ events in the 8 (13) TeV analysis are selected by requiring a photon with a transverse momentum $p_{\rm{T}}$ > 15 (20) GeV, one (two) isolated lepton(s) with large transverse momentum, large missing transverse momentum, and at least four (two) jets, where at least one of them is identified as originating from a $b$-quark. The inclusive and differential cross sections are measured in a fiducial region, defined to be close to the selection requirements. The inclusive fiducial cross sections at 8 TeV and 13 TeV are found to be 139 $\pm$ 18 fb in the single-lepton channel, and 69.3 $\pm$ 4.3 fb in the dilepton channel, in agreement with the theoretical predictions at next-to-leading order in QCD.