Top Friends: Probing Higgs boson associated production with top quarks using the ATLAS detector

Abstract

The Standard Model (SM) of particle physics represents the leading theoretical framework describing fundamental particles and their interactions. The generation of particle masses is explained via the Brout-Englert-Higgs mechanism, which introduces spontaneous symmetry breaking in the electroweak sector and predicts the existence of a scalar boson, known as the Higgs Boson. The Higgs Boson was experimentally confirmed by both the ATLAS and CMS collaborations at the Large Hadron Collider (LHC), CERN in 2012. The exploration of the Higgs Boson's properties, in particular its interactions with fermions, constitutes a pivotal aspect of the post-Higgs discovery era at the LHC. Among these, measuring Higgs production in association with a pair of top quarks ($t\bar{t}H$) offers a unique window into the Higgs-top-Yukawa coupling, the largest predicted Yukawa coupling in the SM. Precise measurements of $t\bar{t}H$ are required to either validate the SM prediction or highlight deviations hinting at possible new physics. This thesis presents a measurement of the $t\bar{t}H$ production cross-section in the $H\rightarrow b\bar{b}$ decay channel. The analysis is performed using 140~\text{fb}^{-1}}~of proton-proton collision data at a centre of mass energy of $\sqrt{s}=13$ TeV, collected with the ATLAS detector between 2015--2018 at the LHC, corresponding to the full Run 2 dataset. The analysis focuses on events with one or two light charged-leptons in the final state. The development and validation of the profile likelihood fit used to extract the signal cross-section is presented. Neural networks employing attention mechanisms and permutation invariant architectures, known as transformers, perform event classification and reconstruction. Network decisions and potential domain bias levels are studied and presented alongside studies to select the optimal set of observables for the profile likelihood fit. Events in excess of the background-only hypothesis are found, equivalent to an observed (expected) discovery significance of 4.6 (5.4) standard deviations, with a measured cross-section of \sigma_{\ttH} = 411^{+101}_{-92}~\fb = 411 \pm 54~\text{(stat.)}~^{+85}_{-75}~\text{(syst.)} \text{fb}\ ,the most precise individual measurement of $t\bar{t}H$ production to date. The measurement agrees with the SM prediction. Differential cross-section measurements with respect to the Higgs Boson transverse momentum are also performed within the simplified template cross-section framework. The analysis provides critical precision in the high $p_{T}^{H}$ regime for future combination and Effective field theory (EFT) interpretations