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 (tt̄H). The analysis uses a 36.1 fb−1 dataset of proton-proton collisions at a center-of-mass energy of √ s = 13 TeV collected with the ATLAS detector, at the Large Hadron Collider during 2015 and 2016. The analysis presented here searches for the tt̄H production in the H → bb̄ 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 tt̄H events against background events being dominated by top pair production with additional b-jets, tt̄+ ≥ 1b. Detailed studies presented here have been performed to estimate the background from tt̄+ ≥ 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 tt̄H signal cross section to the Standard Model expectations, μtt̄H , is found to be μtt̄H = 0.84 +0.64 −0.61. 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 tt̄H signal strength larger than 2.0 is excluded at the 95% confidence level.