First observation of tt̄γγ production and measurement of the tt̄γγ/tt̄γ cross section ratio at √s = 13 TeV using the ATLAS detector

This thesis presents the first observation of top-quark pair production process in association with two photons (tt̄γγ), which is not only rare in the Standard Model but also constitutes an irreducible background to the tt̄H (H → γγ) process. The related tt̄γ process, in which a top-quark pair is produced in association with a single photon, has been well studied, and its inclusive cross section was recently measured by the ATLAS experiment at the Large Hadron Collider (LHC).

The work presented in this thesis focuses on the measurement of the tt̄γγ process with the ATLAS experiment. The measurement is performed in the single-lepton decay channel. The data correspond to an integrated luminosity of 140 fb⁻¹ in proton–proton collisions collected during Run 2 at a centre-of-mass energy of 13 TeV. The measurement is carried out within a fiducial phase space volume defined at particle level using kinematic requirements for the lepton, photons, and jets.

The tt̄γγ production cross section is measured using the profile likelihood method and a Boosted Decision Tree (BDT). The BDT was trained to separate the tt̄γγ signal process from the background processes. Additionally, the ratio of the cross sections of the tt̄γγ and tt̄γ processes is extracted from a simultaneous fit.

The tt̄γγ production cross section is found to be 2.42^{+0.58}_{-0.53} fb and corresponds to an observed significance of 5.2 standard deviations, which constitutes the first observation of this process. Furthermore, the measured ratio of the cross sections of tt̄γγ to tt̄γ yields a value of (3.30^{+0.70}_{-0.65}) X 10^{-3}. However, in the absence of corresponding theoretical predictions, no direct comparison with the experimentally measured values can be performed at present.