Measurement of the tt̄γ production cross section in proton-proton collisions at √s=8 TeV with the ATLAS detector

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of √s=8 TeV with 20.2 fb⁻¹ of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum p_T>15 Ge V, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat. ) ± 17 (syst. ) fb, in good agreement with the theoretical prediction at next-to-leading order of 151±24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.