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 60GeV. 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 5000fb − 1 e + e − collision data at √ s = 240GeV from the Circular Electron Positron Collider (CEPC). This study with 4 b 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 15–60GeV. The sensitivity reach is significantly higher than that can be achieved at the LHC.