Intrinsic Properties of Large CP Violation in the Complex Two-Higgs-Doublet Model

Abstract

We investigate the parameter space supporting large CP violation (CPV) in the complex two-Higgs-doublet model with softly broken $Z_{2}$ symmetry, where the 125~GeV Higgs boson is identified as the lightest neutral Higgs boson $H_1$. Through a comprehensive global scan of Type-I and Type-II models under theoretical, collider, and eEDM constraints, we identify distinct structures that facilitate large CPV. In Type-I, gauge-sector CPV is maximized when the 125~GeV Higgs boson is nearly degenerate with a second neutral scalar. For the ensemble of physically viable points, the predicted eEDM values typically exceed $10^{-31}\,e\cdot\mathrm{cm}$, placing the model largely within the sensitivity of next-generation experiments. Conversely, Type-II models strongly suppress gauge-sector CPV while allowing for nearly maximal CPV in the Yukawa sector. Destructive interference among various contributions allows for $|d_e|$ values as low as $O(10^{-35})\,e\cdot\mathrm{cm}$, resulting in no phenomenologically relevant lower bound. Finally, we uncover the phenomenon of ``hidden CPV'' in the near-alignment limit, characterized by CP-violating mixing between the heavy neutral Higgs bosons governed by the angle $α_3$. We demonstrate that this hidden CPV can be experimentally probed at future colliders via CP-violating Yukawa interactions of $H_2$ and $H_3$, as well as the robust $H_2$-$H_3$-$Z$ coupling.