Probing Electroweak Phase Transition at CEPC via Exotic Higgs Decays with 4b Final States

A strong first-order electroweak phase transition (EWPT) can be induced by light new physics weakly coupled to the Higgs. This study focuses on a scenario in which the first-order EWPT is driven by a light scalar s with a mass between 15 and 60 GeV. A search for exotic decays of the Higgs boson into a pair of spin-zero particles, h → ss, where the s-boson decays into b-quarks promptly is presented. The search is performed in events where the Higgs is produced in association with a Z boson, giving rise to a signature of two charged leptons (electrons or muons) and multiple jets from b-quark decays. The analysis is considering a scenario of analyzing 5000 fb−1 e+e− collision data at √ s = 240 GeV from the Circular Electron Positron Collider (CEPC). This study with 4b final state conclusively tests the expected sensitivity of probing the light scalars in the CEPC experiment. Upper limits are set on the Higgs to double singlet cross-section times branching ration with 95% CL. The ratio of these limits over the SM production cross-section is estimated to be around 5 × 10−4 for the mass range of 15–60 GeV. The sensitivity reach is significantly higher than that can be achieved at the LHC.