Search for same-sign top quark pairs with the ATLAS detector at the LHC

Abstract

In this thesis, a search for same-sign top quark pairs produced according to 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 hightransverse 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. No significant excess with respect to the Standard Model predictions is observed, therefore upper limits are placed on the signal production cross section and on the Wilson Coefficients associated to the considered EFT operators. The observed upper limit at 95% of confidence level on same sign top pairs production cross section times Branching Ratio is 0.86 fb. The observed upper limits at 95% of confidence level on the SMEFT parameters are 0.005, 0.015 and 0.031 ([TeV/Λ]2) for the Wilson Coefficients associated to ORR, O(1)LR and O(8)LR respectively. 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.