Search for the Higgs Boson Produced in Association with Top Quarks and Decaying into Bottom Quarks with the ATLAS Detector

The Higgs boson coupling to the top quarks (top Yukawa-coupling) is one of the most important quantities to be measured experimentally. The top quark has by far the heaviest mass among the Standard Model (SM) particles and with a value approximately $m_{\rm top}\sim {\it v}/\sqrt{2}$ (${\it v}=246$~GeV). The corresponding top Yukawa-coupling is almost unity, which may imply a special relationship between the Higgs boson and the top quark. Higgs boson production in association with a pair of top quarks, $t\bar{t}H$, offers a unique production mode allowing a direct measurement of the top Yukawa-coupling. Although the production cross-section is only $\sim1\%$ of the total Higgs boson production, the $t\bar{t}H$ process offers a distinct signature due to the numerous final state objects that depend on the Higgs boson and $t\bar{t}$ decays. Using the largest branching ratio for the Higgs boson, $H\rightarrow b\bar{b}$ of 58\%, the search for $t\bar{t}H$ production was performed in this thesis. In order to trigger signal events with significant rejections of QCD and multi-jet backgrounds, the $t\bar{t}H(H\rightarrow b\bar{b})$ channel with at least one lepton emitted from $t\bar{t}$ decay was chosen. The difficulties for this channel include a low efficiency in reconstructing all final-state objects, combinatorial ambiguity from multiple $b$-jets in the final state, and large backgrounds from production of $t\bar{t}$ plus additional heavy flavor jets such as $b$-jets and $c$-jets. The $t\bar{t}$ plus jets background is difficult to model and gives essentially the same kinematics as the $t\bar{t}H$ process. The analysis used 36.07~\ifb~of $pp$ collision data at $\sqrt{s}=13$~TeV, collected with the ATLAS detector at the Large Hadron Collider, corresponding to the full $pp$ data sets from 2015-2016 runs. The developed analysis is characterized by the following points, (1) categorize events according to the jet and the $b$-tagged jets with the multiplicity and $b$-tagging working points to maximize the signal sensitivity; (2) utilize two $t\bar{t}H$ reconstruction algorithms, one using both Higgs and $t\bar{t}$ kinematics, while the other using only $t\bar{t}$ kinematics to provide non-biased Higgs kinematics; (3) utilize two discriminant variables for separating the $t\bar{t}H$ signal and the $t\bar{t}+b\bar{b}$ background using a likelihood and a matrix-element method; (4) adopt a multi-variate analysis to obtain the best sensitivity constructed by likelihood and matrix-element discriminants; (5) perform a global fit which simultaneously determines the contributions from the $t\bar{t}H$ signal and the major backgrounds. The $t\bar{t}H$ signal strength, defined as the ratio of the measured $t\bar{t}H$ cross section to the Standard Model expectation is measured to be $\mu = 0.84^{+0.64}_{-0.61}$ assuming a Higgs boson mass of 125~GeV. In the result combining all $t\bar{t}H$ analysis channels, the $t\bar{t}H$ signal strength is measured to be $1.17^{+0.34}_{ -0.32}$. This corresponds to an observed significance of 4.2$\sigma$, while 3.7$\sigma$ was expected from a simulation predicted by the SM. The observed $t\bar{t}H$ production and decay rates are also interpreted in a coupling framework to evaluate the top Yukawa-coupling, which are found to be compatible with the SM prediction.