On top of the Higgs

This PhD thesis details the search for the Standard Model Higgs boson produced in association with a pair of top quarks, $t\bar{t}H$, with the Higgs boson decaying to a $b\bar{b}$ quark pair (and using both the single-lepton and dileptonic $t\bar{t}$ decay mode). The search is carried out using the data collected with the ATLAS detector, in 2015 and 2016, amounting to an integrated luminosity of 36.1 fb$^{-1}$. Several optimisations of the analysis are investigated, among which the development of a separate BDT to distinguish better the contributions from $t\bar{t}$ + heavy-flavour and $t\bar{t}$ + light-flavour backgrounds. In addition, the so-called Jet Vertex Charge algorithm is detailed, which was intended to provide discrimination between jets originating from $\bar{b}$-quark and $b$-quark fragmentation. The performance of this algorithm was calibrated using single-lepton $t\bar{t}$ events, and this performance measurement is also described. The search is summarised in terms of a signal strength measurement, $\mu = 0.84 \pm 0.29 (\mbox{stat.}) ^{+0.57}_{-0.54} (\mbox{syst.})$. This measurement is further combined with measurements of $t\bar{t}H$ production in other Higgs decay modes.