Lepton collider indirect signatures of dark CP-violation

We study an extension of the Standard Model (SM) in which two copies of the SM Higgs doublet are added to the scalar sector. These extra doublets do not develop a vacuum expectation value, hence, they are inert. This essentially leads to a 3-Higgs Doublet Model (3HDM) with 2 inert and 1 active scalar doublets, which we denote as I(2+1)HDM. We allow for CP-violation in the inert sector, where the lightest inert state is protected from decaying to SM particles through the conservation of a $$Z_2$$ symmetry, so that it is a Dark Matter (DM) candidate. For this scenario, we identify a smoking gun signature of dark CP-violation in the form of production thresholds of pairs of inert neutral Higgs bosons at an $$e^+e^-$$ collider.