Identifying Charged Lepton-like Portal Matter at Future Colliders

In the Kinetic Mixing (KM) portal scenario, the interaction of dark matter (DM) with the particles of the Standard Model (SM) is generated by diagrams connecting the familiar photon with its dark sector analog, the dark photon (DP), via loops of particles carrying both dark and SM quantum numbers, \ie, Portal Matter (PM). For the case of sub-GeV DM and DP, these PM states may lie in the $\sim 1-10$ TeV range and be potentially accessible at the HL-LHC as well as at other future lepton and hadron colliders. In perhaps the simplest scenario of this kind, PM consists of just a pair of electrically charged, iso- and color-singlet, vector-like (VL) fermions having opposite dark charges, with an $O(1)$ mass splitting, yielding a finite value for the strength of the KM, \ie, $\epsilon \sim a~ few \times10^{-4}$. The dark Higgs induced mixing of PM states with their SM analogs allows for their decay but can also lead to significant distortions in the expected production properties for the PM at future lepton colliders due to $t$-channel dark Higgs exchange, potentially confusing PM identification. We show that the large set of clean observables available at lepton colliders is more that sufficient to resolve any of these ambiguities. The possibility of the production of like-sign PM fields via $t/u$-channel exchange of the same dark Higgs is also briefly explored.