Cross-Section Measurement of $t\bar{t}\gamma$ Production in $pp$ Collisions at $\sqrt{s} = 8$ and $13$ TeV with the ATLAS Experiment

The cross-section measurement of top-quark pair production in association with a photon ($t\bar{t}\gamma$) probes the electromagnetic coupling of the top quark. The inclusive and differential fiducial cross sections of $t\bar{t}\gamma$ are measured with the data collected by the ATLAS detector from proton-proton collisions at the Large Hadron Collider, at the centre-of-mass energy of $\sqrt{s} = 8~\text{TeV}$, with a corresponding integrated luminosity of $20.2~\text{fb}^{-1}$, and at the centre-of-mass energy of $\sqrt{s} = 13~\text{TeV}$, with a corresponding integrated luminosity of $36.1~\text{fb}^{-1}$. The measurements are performed in the single-lepton final state. Signal events in the $\sqrt{s} = 8~(13)~\text{TeV}$ measurement are selected by requiring one photon with a transverse momentum of $p_\text{T} > 15~(20)~\text{GeV}$, one isolated electron or muon, and at least four jets where at least one of them is originated from a $b$-quark. The fiducial region is defined to be close to the selection requirements. The inclusive cross-sections in $\sqrt{s} = 8$ and $13~\text{TeV}$ are measured to be $139 \pm 7~\text{(stat.)} \pm 17~\text{(syst.)}~\text{fb}$ and $521 \pm 9~\text{(stat.)} \pm 41~\text{(sys.)}~\text{fb}$, respectively, in agreement with the corresponding next-to-leading-order theoretical predictions of $151 \pm 24~\text{fb}$ and $495 \pm 99~\text{fb}$. The differential cross-sections are measured as a function of transverse momentum and absolute pseudorapidity of the photon, in both $\sqrt{s} = 8$ and $13~\text{TeV}$ data-sets, and as a function of the distance between the photon and the lepton in $\sqrt{s} = 13~\text{TeV}$ data-set. All differential measurements are in agreement with the theoretical predictions at next-to-leading-order accuracy.