Search for new physics through Higgs-boson-pair production at the LHC and beyond

This thesis presents a search for resonant production of highly energetic Higgs boson pairs, with each Higgs boson decaying to a b-quark pair, with the ATLAS detector. This analysis uses the complete ATLAS run 2 dataset of 139.0 fb^−1 of proton-proton collisions at a centre-of-mass energy of sqrt(s) = 13 TeV. The expanded dataset, new flavour-tagging techniques, and novel subjet reconstruction algorithms provide better sensitivity than previous studies of the process. Two beyond-the-Standard-Model resonances are chosen as benchmarks for the result: a two-Higgs-doublet model scalar S and Kaluza-Klein (KK) gravitons G_kk predicted in the bulk Randall-Sundrum (RS) model. No significant deviations from Standard Model predictions are found, and upper limits are set on the production cross section of these resonances. A phenomenology study on the analysis strategy of this search in conditions of the future High Luminosity Large Hadron Collider is presented as well. This study shows proof-of-concept improvements in sensitivity by using a deep- learning approach to separate the hh → 4b signal, and discusses the optimization of the search to constrain the Higgs boson self-coupling parameter λ_hhh