Search for the Higgs boson in the $t\bar{t}H(H\rightarrow b\bar{b})$ channel and measurement of the production cross-section of $t\bar{t}b\bar{b}$ with the ATLAS detector at the LHC

Since the discovery of the Higgs boson at a mass around 125 GeV by both ATLAS and CMS collaborations in July 2012, it became crucial to measure its properties, such as its couplings to fermions, and unravel any deviations from the Standard Model (SM) predictions. The top quark Yukawa coupling is the strongest in the fermionic sector and one of the best probes to shed light on phenomena beyond the SM. The Higgs boson production in association with a pair of top quarks and its subsequent decay into a pair of bottom quarks, $t\bar{t}H(H\rightarrow b\bar{b})$, grants direct access to this coupling in addition to the bottom quark Yukawa coupling. Although the $t\bar{t}H$ production was recently established by both ATLAS and CMS combining several channels including $t\bar{t}H(H\rightarrow b\bar{b})$, the latter has never been observed directly. The complexity of the $t\bar{t}H(H\rightarrow b\bar{b})$ final state with a large number of $b$-jets, and the overwhelming background of the top pair production with additional jets ($t\bar{t}+\textrm{jets}$), especially $b$-jets ($t\bar{t}b\bar{b}$), are the major challenges of this channel. Thus, $b$-jet identification referred to as $b$-tagging, and the control of the $t\bar{t}b\bar{b}$ background are key ingredients for this analysis. This thesis presents the search for $t\bar{t}H(H\rightarrow b\bar{b})$ performed with 36.1 $\textrm{fb}^{-1}$ of LHC data collected by the ATLAS detector in 2015 and 2016. Novel techniques to improve the search sensitivity with full Run 2 data are also developed. Particularly, detailed studies are presented including a differential cross-section measurement of the main $t\bar{t}b\bar{b}$ background. The optimization of the $b$-tagging algorithms in view of the High Luminosity LHC upgrade, after the replacement of the ATLAS inner detector, is also presented.