Studying Production of Multiple Higgs Bosons at the Large Hadron Collider

Abstract

The discovery of the Higgs boson (h) in 2012 marked a significant milestone in confirming the Standard Model (SM) as a successful theory. Following this discovery, a key priority of the Large Hadron Collider (LHC) research program is to determine whether the properties of this narrow 125 GeV particle align with SM expectations or if it could be a gateway to physics beyond the Standard Model (BSM). Studying the simultaneous production of two or more Higgs bosons can provide key insights into these questions. This dissertation presents two searches for rare Standard Model processes resulting from proton-proton collisions at a center-of-mass energy of 13 TeV, using data collected with the ATLAS detector at CERN's LHC from 2015 to 2018. The first search investigates Higgs boson pair production (hh) produced in association with a vector boson V (W or Z). Leptonic decays of the vector boson are targeted, with both Higgs bosons decaying to b-quark pairs, resulting in three unique final states based on the number of leptons. The Vhh process involving both SM-like non-resonant and BSM-motivated resonant hh production are explored. Resonant hh production is examined within the context of a neutral heavy scalar resonance H that decays to hh (VH), and a resonant pseudoscalar A boson that decays into a Z and H boson (A → ZH). Upper limits are placed on SM-like non-resonant Vhh production, and constraints are derived on the Higgs boson tri-linear self-coupling and quartic-gauge coupling constants. Upper limits on resonant Higgstrahlung Vhh production cross-section are placed for an H mass in the range of 260-1000 GeV, and for A → ZH production for a scalar A mass in the range of 360-800 GeV with an H boson mass of 260-400 GeV. Overall, a slight excess of observed data over the SM background results in weaker observed constraints than expected. Constraints on A → ZH production are also interpreted in the context of the Two-Higgs-Doublet Model. The second is a search for triple-Higgs boson production (hhh) in the 6 b-quark final state, exploring both SM-like non-resonant and BSM-motivated resonant production. The non-resonant search uniquely constrains hhh production as a function of the trilinear and quartic self-coupling constants. The resonant cascade process X → Sh → hhh is considered primarily, where X and S are two new heavy scalars with masses of up to 1.5 and 1 TeV, respectively. Central to this analysis is the background estimation in the 6b final-state, which is dominated by QCD multi-jet processes. A novel, fully data-driven approach is employed, which features an extrapolation from a lower b-tag to a higher b-tag multiplicity region. In all interpretations, no evidence for hhh production is observed. Upper limits on the hhh production cross-sections are placed at the 95% confidence level and constraints on the trilinear and quartic self-couplings are derived. An upper limit for the Standard Model hhh production cross-section is set at 59 fb at a 95% confidence level.