Measurement of the $t\bar {t}\gamma $ Production Cross Section in Proton-Proton Collisions at $\sqrt {s}=8$ TeV with the ATLAS Detector

The cross section of top-quark pair production in association with a photon in proton–proton collisions at a center-of-mass energy of $\sqrt{s}$ = 8 TeV is measured. The data with a total integrated luminosity of 20.2 fb$^{-1}$ collected by the ATLAS detector at the Large Hadron Collider in 2012 is used. The measurement is performed in the single lepton decay channel. The signal region is defined by the final state of exactly one high-pT lepton, large missing transverse momentum, at least four jets where at least one is being b-tagged and exactly one photon with pT > 15 GeV. The cross section times the branching ratio is determined in a fiducial region defined in terms of the detector acceptance. In addition, the first differential crosssection measurements as a function of photon pT and are presented. The measured fiducial inclusive and differential cross sections are in a good agreement with the next-to-leading order (NLO) prediction.