Hitting the Thermal Target for Leptophilic Dark Matter

We study future lepton collider prospects for testing predictive models of leptophilic dark matter candidates with a thermal origin. We calculate experimental milestones for testing the parameter space compatible with freeze-out and the associated collider signals at past, present, and future facilities. This analysis places new limits on such models by leveraging the utility of lepton colliders. At $e^+e^-$ machines, we make projections using precision $Z$-pole observables from $e^+e^-\to l^+l^- +$ missing energy signatures at LEP and future projections for FCC-ee in these channels. Additionally, a muon collider could also probe new thermal relic parameter space in this scenario via $\mu^+\mu^- \to X +$ missing energy where $X$ is any easy identifiable SM object. Collectively, these processes can probe much all of the parameter space for which DM direct annihilation to $l^+l^-$ yields the observed relic density in Higgs-like models with mass-proportional couplings to charged leptons.