Lepton Collider as a Window to Reheating via Freezing Out Dark Matter Detection

Abstract: We investigate a particle dark matter (DM) scenario where the DM interaction with the Standard Model are mediated by a leptophilic effective operator. Unlike conventional WIMP scenarios where thermal freeze-out occurs in a radiation-dominated Universe, we consider DM freeze-out during a prolonged reheating epoch driven by inflaton decay. The resulting departure from standard cosmology alters the thermal evolution of the dark matter abundance, making it sensitive to the reheating temperature and the history of entropy injection. The leptophilic nature of the interaction, motivated by the absence of DM signals in the current LHC searches, suppresses couplings to quarks and gluons and instead enables viable DM-lepton interactions that remain largely unconstrained. Within this setup, we analyze the mono-Higgs plus missing energy channel at future lepton colliders where the same operator responsible for setting the relic abundance can be directly probed. We perform a detailed signal-background analysis using both polarized and unpolarized beams. Additionally, our results illustrate how collider experiments, when interpreted jointly with relic density constraints, can provide indirect hints of the Universe's thermal history, offering potential insights into the reheating temperature and the dynamics preceding Big Bang Nucleosynthesis.