Search for the associated production of the Higgs boson with a top quark pair in multilepton final states at $\sqrt{s}=13\ TeV$ with the ATLAS detector

The Higgs boson can be produced at proton-proton colliders, such as LHC, via different processes. Among them, the ttH production mode shows the lowest cross-section and the limited statistics makes this search a challenge. The measurement of the coupling between the Higgs boson and the top quark and its comparison with the Standard Model expectation could provide a further confirmation of the validity of the theory, leading to a more complete understanding of the Higgs mechanism. The ttH process, not yet observed, would permit a direct measurement of the quark top-Higgs Yukawa coupling. Moreover, the comparison of this measurement with indirect measurements, already performed via gluon-gluon fusion production mode and Higgs boson decay into photon pairs, has the potential to identify and disentangle new physics effects that can modify the ttH production cross section relative to the SM expectation. In this thesis a search, performed using $13.2\ fb^{-1}$ of data recorded by the ATLAS detector in 2015 and 2016 at a centre-of-mass energy of 13 TeV, is presented. The analysis is designed to search for the Higgs boson, produced in association to a top quark pair, through its decays into $WW^*$, $ZZ^*$ and $\tau\tau$. These channels present a quite complex signature, characterized by a high jet multiplicity and a different number of light leptons in the final state, according to the decay modes of the top quark pair and Higgs boson. In particular, the analysis object of this work, requires the presence of three leptons (electrons or muons) in the final state. For this channel, I developed the estimation of the background due to non-prompt leptons and I elaborated a study based on the full event kinematic reconstruction, with the goal of reducing the background from vector bosons produced in association to a top quark pair (ttV). The results obtained from the $3\ell$ channel and from the combination among all the multilepton channels are compared with the SM expectation, assuming the background-only hypothesis. The cross-section measured in the $3\ell$ channel leads to an observed (expected) significance of $2.0\ \sigma$ ($0.9\ \sigma$). For the combination, an observed (expected) significance of $2.8\ \sigma$ ($1.1\ \sigma$) is obtained.