Search for the production of a pair of highly Lorentz-boosted Higgs bosons via vector-boson fusion in final states with four bottom quarks with the ATLAS experiment

A search for the production of a pair of Higgs bosons ($HH$) via vector-boson fusion (VBF) in the Lorentz-boosted kinematic regime with subsequent decay of the Higgs bosons into four bottom quarks is presented. The analysis is based on 140~fb$^{-1}$ of proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider (LHC), at a center-of-mass energy of $\sqrt{s} = 13$~TeV. The results were interpreted in terms of both non-resonant and resonant production of Higgs-boson pairs. Non-resonant VBF $HH$ production is predicted by the Standard Model (SM); the production cross-section is, however, significantly enhanced for anomalous, non-SM values for the trilinear Higgs-boson self-coupling and the quartic coupling between two Higgs bosons and two vector bosons, parametrised as $\kappa_{\l}$ and $\kappa_{2V}$, respectively. Resonant VBF $HH$ production is predicted only by extensions of the SM. In this thesis, the resonant $HH$ production via the decay of a new heavy scalar particle originating from the vector-boson scattering was considered. No significant deviation of the data from the expected background was observed. Hence, in the non-resonant search, limits on the $\kappa_{2V}$ and $\kappa_{\l}$ couplings were set at $95\%$ confidence level. In the resonant search, upper limits on the signal cross-section were derived for intermediate masses ranging from 1.0~TeV to 5.0~TeV under the assumptions of a narrow- and broad-resonance widths. In the model-dependent broad-width scenario, a composite Higgs benchmark has been considered. In addition to these results, which have been published in 2024, the relevant details of a successive on-going ATLAS analysis using additionally data collected in 2022 and 2023 at $\sqrt{s} = 13.6$~TeV and using more modern tools to identify the $H \to b\bar{b}$ decays is discussed here. Finally, a proof-of-principle study using a transformer-based machine-learning algorithm to discriminate signal from background events in triple Higgs-boson searches is also shown in this thesis. The even rarer production of three Higgs bosons has been gaining significant interest since it provides unique access to the quartic Higgs-boson self-coupling.