Mixed CP Violation and Natural Alignment in 2HDMs

We present a new form of CP violation (CPV) that can be realised in Two-Higgs Doublet Models (2HDMs) and was studied recently in [1]. By examining the vacuum manifold of a generic (convex) 2HDM potential, we identify scenarios that exhibit Mixed Spontaneous and Explicit CP Violation (MCPV), in which at least two non-degenerate CP-violating local minima coexist. We illustrate how this identification is achieved at the tree level by determining the magnitude and phase of a novel complex parameter, which we call $r_{\rm CP}$. Since explicit CP Violation vanishes in 2HDMs where SM Higgs alignment is enforced through global continuous symmetries, we investigate how to maximise CPV in such scenarios by introducing soft or explicit breaking of the relevant symmetries. In doing so, we derive upper bounds on key CP-violating parameters that characterise misalignment with the SM, subject to constraints from the non-observation of the electron electric dipole moment. Finally, we delineate the region of the CP-violating parameter space in such constrained 2HDMs that can be further tested at the CERN Large Hadron Collider.