Probing the top-quark coupling to the photon through the cross-section measurement of <i>t<span style=

Abstract

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 of 8 TeV with 20.2 fb^-1 of data and at 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 <i>t<span style="text-decoration:overline;">t</span>γ</i> events in the 8 (13) TeV analysis are selected by requiring a photon with a transverse momentum <i>p</i>_{<small>T</small>} > 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 <i>b</i>-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 ± 18 fb in the single-lepton channel, and 69.3 ± 4.3 fb in the dilepton channel, in agreement with the theoretical predictions at next-to-leading order in QCD.