Measurement of the $t\bar{t}\gamma$ production cross section in proton--proton collisions at $\sqrt{s} = 8\;\mathrm{Te\kern -0.1em V}$ 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 $\sqrt{s} = 8\;\mathrm{Te\kern -0.1em V}$ 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 $p_\mathrm{T} > 15\;\mathrm{Ge\kern -0.1em V}$, 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 $139 \pm 7\,(\mathrm{stat.\!}\,) \pm 17\,(\mathrm{syst.\!}\,)\,\mathrm{fb}$, in good agreement with the theoretical prediction at next-to-leading order of $151 \pm 24$ fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.