This paper presents the first observation of top-quark pair production in association with two photons ($t\bar{ t}\gamma\gamma$). The measurement is performed in the single-lepton decay channel using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider. The data correspond to an integrated luminosity of 140 fb$^{ -1}$ recorded during Run 2 at a centre-of-mass energy of 13 TeV. The $t\bar{ t}\gamma\gamma$ production cross section, measured in a fiducial phase space based on particle-level kinematic criteria for the lepton, photons, and jets, is found to be $2.42^{ +0.58}_{ -0.53}\, \text{ fb}$, corresponding to an observed significance of 5.2 standard deviations. Additionally, the ratio of the production cross section of $t\bar{ t}\gamma\gamma$ to top-quark pair production in association with one photon is determined, yielding $(3.30^{ +0.70}_{ -0.65})\times 10^{ -3}$.
Categories: hep-ex