Lepton collider as a window to reheating via freezing in dark matter detection. Part I

We propose a methodology to infer the reheat temperature ($T_{\rm RH}$) of the Universe from the collider signal of freezing in dark matter (DM). We demonstrate it for the mono-$\gamma$ signal at the electron-positron colliders, which indicates to a low-scale $T_{\rm RH}$, after addressing observed DM abundance, BBN, and other relevant constraints. The method can be used to correlate different reheating dynamics, DM models, and collider signals.