Investigation of the discovery potential of a Higgs boson in the $t\bar{t}H^0, H^0 \to b\bar{b}$ channel with the ATLAS experiment

The production of a light Standard Model Higgs boson in association with a top-quark pair at the Large Hadron Collider (LHC) is studied in a simulation of the multipurpose ATLAS experiment. The Higgs boson is assumed to decay into a $b\bar{b}$ pair, and the top-quark pair to decay semi-leptonically. The main background process for this channel is the production of $t\bar{t}$ events, which can be divided into reducible and irreducible components. In the process of generating these components separately, an overlap of events is created through the addition of b-quarks to $t\bar{t}$ events via parton showering in the $t\bar{t}X$ sample. These events are already included in the matrix-element cross-section calculation for the $t\bar{t} b\bar{b}$ sample. A new procedure for the overlap removal is presented in this thesis. Two analyses are studied, where one aims at the full reconstruction of the final state using a cut-based approach. Recently, this way of reconstructing the $t\bar{t} H^0 (H^0 \to b\bar{b})$ process has been found to be extremely challenging. The other analysis is based on a new method employing state-of-the-art jet reconstruction and decomposition techniques where the $t\bar{t}$ pair and the Higgs boson are required to have large transverse momenta and can therefore be reconstructed as massive Higgs and top jets. A recent phenomenological study has shown that the $t\bar{t} H^0$ process can be recovered as a promising search channel for a low mass Standard Model Higgs boson around 120 GeV using this approach. Finally, to enhance the sensitivity of the $t\bar{t} H^0$ channel, a combination of the two analyses is presented.