SEARCH FOR THE STANDARD MODEL HIGGS BOSON PRODUCED IN ASSOCIATION WITH TOP QUARK PAIRS IN MULTI-LEPTONIC FINAL STATES WITH THE ATLAS DETECTOR AT THE LHC

The search for the Standard Model (SM) Higgs boson produced in asso- ciation with top quarks - known as ttH production - plays a crucial role in the Large Hadron Collider (LHC) physics programme, as it allows a direct measurement of the Higgs field Yukawa coupling to the heaviest fermion and can constrain effects of new physics beyond the Standard Model in the top coupling sector. This thesis presents a search for the ttH production in an inclusive mul- tileptonic final state, with a proton-proton collision dataset corresponding to an integrated luminosity of Int( L dt ) = 36.1 fb 121, collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy 1as=13TeV. The final state is characterised by high jet multiplicity, and the presence of several electrons and muons, as well as hadronically decaying tau leptons. The multiplicity of these physics objects allows the definition of several categories to enhance the sensitivity of the analysis. The particular focus of my work lies on the final state where exactly two light leptons with the same electric charge and no hadronic taus are found - indicated as 2l SS 0\u3c4had - for which I developed a novel technique to estimate the reducible background of non-prompt (fake) electrons and muons. Boosted decision tree algorithms are trained to discriminate the ttH signal events from the two major background processes in this channel: ttV (V=W,Z) and events with fake leptons. A fit of our model to the observed data is performed, and the results are interpreted using a frequentist approach. A best-fit value for the strength of the ttH production cross section with respect to the Standard Model expectation of \u3bc = 1.5+0.7 is observed. The observed sensitivity of this search corresponds to a 2.7\u3c3 excess of events above the SM background-only hypothesis, with an expected median sensitivity of 1.9\u3c3 for a model where the SM ttH production is assumed. Combination with the other categories of the ttH to multi-leptons analysis eventually leads to a signal strength of \u3bc = 1.6+0.5, with an observed (expected) sensitivity of 4.1\u3c3 (2.8\u3c3) above the SM background-only hypothesis. This indicates the strongest evidence to date for the ttH production mode. Furthermore, I present a study on improvements to the ATLAS track re- construction algorithm to enhance its performance in environments with high density of tracks, such as the core of boosted hadronic jets and hadronically decaying tau leptons