Search for the Production of a Standard Model Higgs Boson in Association with Top-Quarks and Decaying into a Pair of Bottom-Quarks with 13 TeV ATLAS Data

This thesis presents the search for the Standard Model Higgs boson produced in association with a pair of top-quarks ($t\bar{t}H$). The analysis uses a $36.1~\textnormal{fb}^{-1}$ dataset of proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13$ TeV collected with the ATLAS detector, at the Large Hadron Collider during $2015$ and $2016$. The analysis presented here searches for the $t\bar{t}H$ production in the $H \rightarrow b\bar{b}$ decay mode. The selected events contain either one or two leptons from the decay of the top-quark pair. In order to improve the sensitivity of the search, events are split in regions according to the number of jets and how likely these events are to contain $b$-jets. Methods based on multivariate techniques were developed and applied in the signal-enriched regions to discriminate $t\bar{t}H$ events against background events being dominated by top pair production with additional $b$-jets, $t\bar{t}+\geq 1b$. Detailed studies presented here have been performed to estimate the background from $t\bar{t}+\geq 1b$. Moreover, misidentification of leptons causes background events in the analysis samples which are estimated using a data-driven technique based on the Matrix Method. All analysis regions are combined in a statistical model using a profile likelihood fit to constrain the background predictions and reduce the systematic uncertainties. For a Higgs boson mass of $125$ GeV, the ratio of the measured $t\bar{t}H$ signal cross section to the Standard Model expectations, $\mu_{t\bar{t}H}$, is found to be \begin{equation*} \mu_{t\bar{t}H} = 0.84^{+0.64}_{-0.61}. \end{equation*} \noindent An excess of events over the expected Standard Model background is found with an observed (expected) significance of 1.4 (1.6) standard deviations. A $t\bar{t}H$ signal strength larger than $2.0$ is excluded at the $95\%$ confidence level.