Measurement of top quark pair production in association with bosons in multi-leptonic final states using ATLAS Run 2 data

In this thesis, two measurements of the top quark pair production in association with a boson in multi-leptonic final states are presented. Both have been performed using the 140 $fb^{−1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV collected with the ATLAS detector during the Run 2 of the Large Hadron Collider. The first measurement corresponds to a search for physics beyond the Standard Model (SM) using an Effective Field Theory (EFT) approach in the high-mass $t\bar{t}l^{+}l^{-}$ regime. It is the first dedicated measurement, constraining the EFT four-fermion operators in the top quark sector. The data are measured to agree well with the SM expectation. New constraints are set on the flavour inclusive four-fermion EFT operators, improving the current limits. Additionally, the first flavour dependent measurement of the four-fermion operators in the $t\bar{t}l^{+}l^{-}$ vertex is performed, measuring the $t\bar{t}e^{+}e^{-}$ and $t\bar{t}\mu ^{+}\mu ^{-}$ rates separately, as well as measuring the relative electron muon rate. These flavour dependent measurements give access to a potential lepton flavour universality violation in the EFT operators. Similarly to the flavour inclusive case, no significant deviations from the SM expectations are observed for the electron and muon EFT operators. The second measurement targets the $t\bar{t}H$ process, having a direct sensitivity to the top Yukawa coupling. The results are compatible with the SM within $2\sigma$ and measure the $t\bar{t}H$ process in a multi-leptonic final state with a significance
of $3.2\sigma$. Additionally, this measurement corresponds to the first ATLAS differential measure- ment of $t\bar{t}H$ in the multi-leptonic final state, bringing sensitivity to the Higgs self-coupling or the Higgs CP properties.