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 association with top quarks - known as $t\bar{t}H$ 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 $t\bar{t}H$ production in an inclusive multi-leptonic final state, with a proton-proton collision dataset corresponding to an integrated luminosity of $\int L dt = 36.1~\mathrm{fb}^{−1}$, collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$. 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. My focus lies on the final state where exactly two light leptons with the same electric charge and no hadronic taus are found - indicated as $2lSS0\tau_{had}$ - 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 $t\bar{t}H$ signal events from the two major background processes in this channel: $t\bar{t}V$ $(V=W,Z)$ 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 $t\bar{t}H$ production cross section with respect to the Standard Model expectation of $\mu = 1.5 ^{+0.7}_{-0.6}$ is observed. The observed sensitivity of this search corresponds to a $2.7\sigma$ excess of events above the SM background-only hypothesis, with an expected median sensitivity of $1.9\sigma$ for a model where the SM $t\bar{t}H$ production is assumed. Combination with the other categories of the $t\bar{t}H$ to multi-leptons analysis eventually leads to a signal strength of $\mu = 1.6 ^{+0.5}_{-0.4}$, with an observed (expected) sensitivity of $4.1\sigma$ ($2.8\sigma$) above the SM background-only hypothesis. This indicates a strong evidence for the $t\bar{t}H$ production mode. Furthermore, I present a study on improvements to the ATLAS track reconstruction 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.