Thesis defense of Noemi Cavalli

In this thesis, a search for same-sign top quark pairs produced accordingto the Standard Model Effective Field Theory (SMEFT) is presented. The analysis is carried out within the ATLAS Collaboration using collision data at a center-of-mass energy of √s = 13 TeV, collected by the ATLAS detector during the Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb−1. Three SMEFT operators are considered in the analysis, namely ORR, O(1) LR, and O(8) LR. The signal associated to same-sign top pairs is searched in the dilepton channel, with the top quarks decaying via t −→ W+b −→ ℓ+νb, leading to a final state signature composed of a pair of high-transverse momentum same-sign leptons and b-jets. Deep Neural Networks are employed in the analysis to enhance sensitivity to the different SMEFT operators and to perform signal-background discrimination. This is the first result of the ATLAS Collaboration concerning the search for same-sign top quark pairs production in proton-proton collision data at √s = 13 TeV, in the framework of the SMEFT.