Probing CP Violation in the Complex 2HDMS with a 95 GeV Scalar at Future $e^+e^-$ Colliders

Abstract

In this study, we investigate the prospects for probing CP violation in the complex singlet extension of the two-Higgs-doublet model (2HDM) at future electron-positron colliders. In extended scalar sectors, cancellations among different contributions to electric dipole moments (EDMs) can relax the stringent constraints on CP-violating phases, potentially bringing them in the current reach of collider experiments. We consider a scenario in which both the 125 GeV Higgs boson and the additional scalar associated with the reported 95 GeV excess are CP-mixed states. We probe the CP-violating phase of the 95 GeV scalar through its production via Higgsstrahlung and subsequent decay into a tau-lepton pair, using a CP-sensitive forward-backward asymmetry in hadronic and semileptonic tau-decay channels. We estimate the sensitivity at future lepton colliders, namely the ILC, FCC-ee, and CEPC, and investigate the impact of the Standard Model Drell-Yan background. We find that the hadronic channel offers greater sensitivity than the semileptonic channel. We also compare our results with existing constraints and future projections for CP violation in the 125 GeV Higgs sector and briefly discuss the implications for electroweak baryogenesis. Our results highlight the potential of future lepton colliders to probe CP violation in extended scalar sectors while remaining consistent with current EDM constraints.