Identifying Charged Lepton-like Portal Matter at Future Colliders

TL;DR

This work analyzes charged lepton-like Portal Matter (PM) required for kinetic-mixing portals, focusing on a minimal model with TeV-scale vector-like leptons of opposite dark charge that mix with SM leptons via a dark Higgs. The study shows that t-channel dark-Higgs exchange in l+l- -> E+E- can significantly modify production observables, making lepton colliders essential for unambiguous PM identification; observables include σ, A_FB, and polarization asymmetries A_LR and A_T, which together distinguish PM from vanilla VL leptons and from hypothetical Z'-mediated scenarios. It also highlights like-sign PM production in l± l± collisions, with a strong y^4 dependence, as a clean, background-suppressed channel. The results indicate lepton colliders can robustly determine PM properties and thus illuminate the UV completion of the kinetic-mixing portal, while hadron colliders offer limited access to the t-channel PM effects.

Abstract

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, $ε\sim a~ few \times10^{-4}$. 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.