Searches for Higgs pair production in the 4 b-jet final state with the ATLAS Detector at the LHC

The Standard Model of Particle Physics is the prevailing theory for describing the interactions of all observed fundamental particles and three of the four known fundamental interactions. However, despite its profound success, the Standard Model fails to explain some observations, such as dark matter and matter-antimatter asymmetry. Additionally, incorporating Einstein’s theory of general relativity has proven difficult. Many proposed extensions to the Standard Model resolve these and other open questions through modifications of the Higgs potential or through new particles which interact with the Higgs boson. This dissertation presents a search for Higgs boson pair production in the signal rich 4 b-jet final state using 136 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider. This search consists of two separate analysis strategies targeting resonant and non-resonant Higgs boson pair production. The resonant search provides sensitivity to the resonant decay of new massive scalar and spin-2 particles predicted by many extensions to the Standard Model. The non-resonant search is directly sensitive to the Higgs potential via the Higgs boson self-coupling and the four-point Higgs to vector boson couplings. No significant excesses are observed in the resonant search. World-leading upper limits are placed on cross-sections for new massive scalar and spin-2 resonances decaying to two Higgs bosons. These limits exclude a wide range of Kaluza-Klein graviton models and provide leading sensitivity amongst the ATLAS Higgs pair searches for masses above 600 GeV. In the non-resonant search, no evidence for Standard Model Higgs production is found. An observed (expected) upper limit of 5.49 (6.73) times the Standard Model cross-section is set. Limits are placed on the Higgs self-coupling and the four-point Higgs to vector boson couplings, with observed (expected) constraints of κλ ∈ [-4.7, 12.2] ([-4.1, 10.4]) and κ2V ∈ [0.02, 2.05] ([0.01, 2.06]), respectively. These results are then reinterpreted in the more general Higgs Effective Field Theory and Standard Model Effective Field Theory frameworks. The resulting limits are found to provide leading sensitivity within the HH channel for several parameters and benchmark signals.