A relook at a low reheating temperature: Freeze-in versus Freeze-out

Abstract: A lowering of the post-inflation reheating temperature $T_{\rm RH}$ has attracted much attention as a means to facilitate obtaining the correct Dark Matter (DM) relic abundance. We take a relook at the paradigm under the assumption that the inflaton decay is essentially instantaneous, with the thermal bath coming into existence only at a relatively low temperature. We demonstrate that, contrary to the popular misconception, the relevant scale is not necessarily the DM mass itself, but the characteristic temperature at which the dominant DM production processes are thermally efficient. With $T_{\rm RH}$ setting the maximum temperature available for thermal production, and thereby determining the accessible stages of the production history, lowering $T_{\rm RH}$ tends to reduce the relic abundance. While this effect could, in principle, be offset by altering the couplings defining the theory, there exists a minimum $T_{\rm RH}$---determined by the mass spectrum---below which the relic abundance falls to minuscule levels, whether the mechanism be freeze-in or freeze-out.