Observation of the associated production of the Higgs boson with a top quark pair with the ATLAS experiment at the LHC: A study in the boosted regime of the $H \rightarrow b\bar{b}$ decay channel

Probing the coupling of the Higgs boson to the heaviest known fermion, the top quark, is crucial for testing the Standard Model (SM) and for constraining new physics models. This thesis presents a search for the $t\bar{t}H$ process which gives direct access to this Yukawa coupling. The analysis is optimised for the Higgs decaying to a pair of bottom quarks and uses 36.1 fb$^{-1}$ of data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector. One of the main challenges of this analysis is the combinatorial ambiguity from the many jets in the final state which makes it difficult to reconstruct the Higgs boson. The boosted analysis specifically targets final states with high transverse momentum in which the decay products of the Higgs boson and/or hadronically decaying top quark are produced collimated into large jets. This gives access to different kinematics and a simplified combinatorial background. To select events rich in $t\bar{t}H$, we apply cuts on the number of jets and $\textit{b}$-jets (jets tagged as containing $\textit{b}$-hadrons). The boosted analysis also requires two large jets in each event which are constructed with the reclustering method. Since the $t\bar{t}$ background is overwhelmingly large compared to the $t\bar{t}H$ signal, multivariate techniques are used to discriminate between signal and background events. A Boosted Decision Tree (BDT) is used with eight variables including the Higgs candidate mass. The boosted analysis is combined with the resolved selection to obtain a ratio of the measured cross-section to the SM expectation of $\mu_{t\bar{t}H}=0.84 ^{+0.64} _{-0.61}$. This corresponds to a significance of $1.4\sigma$, with an expectation of $1.6\sigma$. A $t\bar{t}H$ signal strength larger than 2.0 is excluded at the $95\%$ confidence level. The analysis in the $b \bar{b}$ decay channel is combined with three other $t\bar{t}H$ searches optimised for the multilepton, $\gamma \gamma$, and $\textit{ZZ}$ decay modes. The combination in which all analyses use 36.1 fb$^{-1}$ of data leads to a significance of 4.2$\sigma$. This constitutes evidence for $t\bar{t}H$ production and corresponds to a cross-section of $\sigma_{t\bar{t}H} = 590^{+160}_{-150} \ \text{fb}$ which is compatible with the SM prediction of $507^{+35}_{-50}$ fb. The combination is repeated with the $\gamma \gamma$ and $ZZ$ decay channels updated to include ATLAS data from 2017 and inclusion of the Run I dataset. This results in an observed (expected) significance of 6.3$\sigma$ (5.1$\sigma$) which marks the first direct observation of the Higgs coupling to the top quark.