Towards UV-Models of Kinetic Mixing and Portal Matter V: Indirect Probes of the New Physics Scale

Kinetic mixing of the dark photon, the gauge boson of a hidden $U(1)_D$, with the Standard Model (SM) gauge fields to induce an interaction between ordinary matter and dark matter (DM) at 1-loop requires the existence of portal matter (PM) fields having both dark and SM charges. As discussed in earlier work, these same PM fields can also lead to other loop-level mechanisms besides kinetic mixing that can generate significant interactions between SM fermions and the dark photon in a manner analogous to those that can be generated between a Dirac neutrino and a SM photon, \ie, dark moments. In either case, there are reasons to believe, \eg, due to the RGE running of the $U(1)_D$ gauge coupling, that PM fields may have $\sim$ TeV-scale masses that lie at or above those directly accessible to the HL-LHC. If they lie above the reach of the HL-LHC, then the only way to possibly explore the physics at this high scale in the short term is via indirect measurements made at lower energies, \eg, at lepton colliders operating in the $m_Z$ to 1 TeV range. In particular, processes such as $e^+e^- \to \gamma+$DM or $e^+e^-\to \bar ff$, where $f$ is a SM fermion, may be most useful in this regard. Here we explore these possibilities within the framework of a simple toy PM model, introduced in earlier work, based on a non-abelian dark gauge group completion operating at the PM scale. In the KM setup, we show these efforts fail due to the inherently tiny cross sections in the face of substantial SM backgrounds. However, in the case of interactions via induced dark moments, since they necessarily take the form of higher dimensional operators whose influence grows with energy, we show that access to PM-scale information may become possible for certain ranges of the toy model parameters for both of these $e^+e^-$ processes at a 1 TeV collider.