Evidence for the production of a Higgs boson in association with two top quarks with the ATLAS detector: A search in the $H\rightarrow b\bar{b}$ channel and in combination with other Higgs boson decays at $\sqrt{s}=$ 13 TeV

In this thesis, the search for the production of the Higgs boson in association with two top quarks is presented. The main focus of this work is on the analysis optimised for the decay of the Higgs boson to a b-quark pair. The analysis is performed using 36.1 fb$^{-1}$ of pp collision data at a centre of mass energy √ s = 13 TeV collected by the ATLAS detector at the Large Hadron Collider during 2015 and 2016. The signal strength of t$\bar{t}$H in relation to the Standard Model prediction for a Higgs boson with a mass of 125 GeV is measured to be μt$\bar{t}$H = 0.87$_{-061}^{+0.64}$, with signal strengths greater than 2.0 excluded at the 95% confidence level. The combination of this analysis with searches targeting additional Higgs boson decay modes is subsequently presented. The measured signal strength in relation to the Standard Model prediction is μ$\bar{t}$H = 1.2±0.3. This corresponds to an observed (expected) significance for t$\bar{t}$H of 4.2σ (3.8σ), constituting evidence for the t$\bar{t}$H production mode. Finally, a study into the ability to observe and model the colour connection of b-quarks in t$\bar{t}$H H → bb̄ and t$\bar{t}$ + jets events is presented. The jet pull angle observable is used to investigate the effect of colour connection on jet substructure. Such an observable is found to be sensitive to the underlying colour structure in events, showing differences between b-quarks which decay from a colour singlet in comparison to a colour octet. However, the effect is found to be small and a larger dataset is required to measure the effect in t$\bar{t}$H events.