Probing a Scalar Singlet-Catalyzed Electroweak Phase Transition with Resonant Di-Higgs Production in the $4b$ Channel

Abstract: 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 $bb\gamma\gamma$ and $4\tau$ channels for heavy Higgs boson mass $m_2$ below 500 GeV and superior to these channels for $m_2 &gt; 500$ GeV. With 3 ab$<sup>{-1}$ of integrated luminosity, it is possible to discover the heavy Higgs boson in the $4b$ channel for $m_2&lt; 500$ GeV in regions of parameter space yielding a strong first order electroweak phase transition and satisfying all other phenomenological constraints.