Probing strong first-order electroweak phase transition scenarios in two-Higgs-doublet model with FCC-ee/CEPC

We investigate the potential of future electron–positron colliders, such as FCC-ee and CEPC, to test a strong first-order electroweak phase transition (SFOEWPT) in the CP-conserving type I two-Higgs-doublet model. We confront the SFOEWPT-favored parameter space with projected electroweak precision and Higgs measurements, as well as searches for additional scalar states. We show that radiative corrections to e$^+$e$^-$ -> $hZ$ production can generate deviations in the cross section that exceed the anticipated sub-percent precision at lepton colliders, even when LHC and Z-pole measurements remain consistent with the SM. Precision Higgs-strahlung measurements therefore provide an indirect and sensitive probe of electroweak-scale BSM effects.