$t\bar{t}H$ Production Measurement in the $H\,{\rightarrow}\,b\bar{b}$ Decay Channel with ATLAS at $13\,\text{Te}\kern-0.11em\text{V}$ and System Tests for the Pixel Detector

The Yukawa coupling between the Higgs boson and the top quark – the heaviest elementary particle in the Standard Model of particle physics – is one of the fundamental parameters of the theory. With a value close to unity, it may play a unique role in electroweak symmetry breaking. Additionally, it is central to the shape of the Higgs potential and possible models of vacuum decay, while at the same time being highly sensitive to physics processes beyond the Standard Model. The top Yukawa coupling is only directly accessible in processes involving the associated production of a Higgs boson with one or more top quarks. This thesis presents an analysis of the associated production of a top quark pair with a Higgs boson in the bottom–anti-bottom Higgs boson decay channel and the leptonic top quark decay modes. The analysis uses the full Run 2 dataset of $140\,\text{fb}^{-1}$ collected between 2015 and 2018 at a centre-of-mass energy of $13\,\text{Te}\kern-0.11em\text{V}$ with the ATLAS detector at the Large Hadron Collider. Since a previous analysis using the same dataset has been published already, a focus is placed on improvements included in the legacy re-analysis presented in this thesis. Herein, special attention is given to the development of permutation-invariant Transformer neural networks to suppress dominant background processes and reconstruct the Higgs boson from its decay products for a differential cross-section measurement conducted as part of the analysis. The observed significance against a background-only hypothesis is $4.6\,\sigma$ compared to an expected significance of $5.4\,\sigma$. The measured inclusive cross-section of $411^{+101}_{-~~92}\,\text{fb}$ for a Higgs boson mass of $125.09\,\text{Ge}\kern-0.11em\text{V}$ is consistent with the Standard Model prediction and represents the most precise single-channel measurement of the signal process to date. The identification of hadronic jets originating from bottom quarks is of central importance to the analysis presented in this thesis since four such jets are expected in the leading-order final state of the signal process. The Pixel detector situated at the core of the ATLAS detector is crucial for identifying the characteristic displaced secondary vertices of these jets. Therefore, a second focus of this thesis lies in the development of automated system tests for the Pixel detector to ensure continued good detector performance for future physics analyses using Run 3 data.