Probing a scalar singlet-catalyzed electroweak phase transition with resonant di-Higgs boson production in the 4b channel

We investigate the prospective reach of the 14 TeV HL-LHC for resonant production of a heavy Higgs boson that decays to two SM-like Higgs bosons in the 4b final state in the scalar singlet extended Standard Model. We focus on the reach for choices of parameters yielding a strong first order electroweak phase transition. The event selection follows the 4b analysis by the ATLAS Collaboration, enhanced with the use of a boosted decision tree method to optimize the discrimination between signal and background events. The output of the multivariate discriminant is used directly in the statistical analysis. The prospective reach of the 4b channel is compatible with previous projections for the bbγγ and 4τ channels for heavy Higgs boson mass m₂ below 500 GeV and superior to these channels for m₂>500  GeV. With 3  ab⁻¹ of integrated luminosity, it is possible to discover the heavy Higgs boson in the 4b channel for m₂<500  GeV in regions of parameter space yielding a strong first order electroweak phase transition and satisfying all other phenomenological constraints