Enlightened Top Quark: Measurements of the $t\bar{t}\gamma$ Cross Section and of its Spectrum in Transverse Energy of the Photon in the Single Lepton Channel at $\sqrt{s} = 7 \ \rm TeV$ in 4.59 fb$^{-1}$ of $pp$ Collision Data Collected with the ATLAS Detector

The top-photon electromagnetic couplings can be probed via the analysis of the production of top quark pairs ($t\bar{t}$) in association with a photon ($\gamma$). A dataset of events with final-states containing jets, missing transverse momentum, one isolated electron or muon and an energetic photon is selected out of $4.59\pm0.08 \text{fb}^{-1}$, of proton--proton collisions at a centre-of-mass energy of $7\ \rm TeV$ recorded by the ATLAS detector at the CERN Large Hadron Collider. In total $140$ and $222$ $t\bar{t}\gamma$ candidate events are observed in the electron and muon channels, respectively. They are to be compared to an expectation of $140$ and $222$ background events in the single-electron and single-muon channels respectively. A first observation of $t\bar{t}\gamma$ state, combining both electron and muon channels, is reported, with $t\bar{t}\gamma$ events being separated by $5.3$ standard deviations from the background only hypothesis. The $t\bar{t}\gamma$ production cross section times the branching ratio (BR) of the single-lepton decay channel, as well as its spectrum in transverse energy of the photon, are measured in a fiducial phase-space within the detector acceptance. The measured cross section is $\sigma_{t\bar{t}\gamma}\times \text{BR} = 63 \pm 8\ \text{(stat.)} \ ^{+16}_{-13}\ \text{(stat.)}\ \pm 1 \ \text{(lumi.)} \ \text{fb}$ per lepton flavour, which is in good agreement with the leading-order theoretical calculation normalised to the next-to-leading-order theoretical prediction of $48\pm10\ \text{fb}$.