Search for VHH production in the HH → bƀbƀ and V → j j final state with CMS Run II data

This thesis presents a search for Higgs boson pair (HH) production in association with a vector boson V (W or Z), focusing on the bƀbƀ decay channel of HH and the hadronic decays of the V boson. At the time of writing, this analysis is the only VHH search conducted in a fully hadronic final state. The analysis is based on proton-proton collision data at a center-of-mass energy of 13 TeV, collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb−1 . Machine learning based background modeling and signal extraction methods, together with a new data-driven background systematic uncertainty estimation strategy, are employed to improve the sensitivity of the search. The observed (expected) 95% CL upper limits on the pp → VHH production cross section are 367 (220) times the SM prediction and 70 (58) times the κλ = 5.5 prediction. When combined with all VHH channels in the HH → bƀbƀ final state, the observed (expected) upper limits are 294 (124) times the Standard Model prediction and 43 (22) times the κλ = 5.5 prediction. In the combined result, the observed (expected) 95% CL constraints on the coupling modifiers are −37.7 < κλ < 37.2 (−30.1 < κλ < 28.9) and −12.2 < κ2V < 13.5 (−7.2 < κ2V < 8.9), assuming all other couplings to be standard-model-like. In the context of the inclusive HH combination, the VHH channel with HH → bƀbƀ achieves a sensitivity comparable to other searches in the range of 4 < κλ < 7.