On ultraviolet freeze-in dark matter during reheating

The absence of any confirmative signals from extensive DM searching motivates us to go beyond the conventional WIMPs scenario. The feebly interacting massive particles (FIMPs) paradigm provides a good alternative which, despite of its feebly interaction with the thermal particles, still could correctly produce relic abundance without conventional DM signals. The Infrared-FIMP based on the renormalizable operators is usually suffering the very tiny coupling drawback, which can be overcome in the UltraViolet-FIMP scenario based on high dimensional effective operators. However, it is sensitive to the history of the very early Universe. The previous works terminates this sensitivity at the reheating temperature T-RH. We, motivated by its UV-sensitivity, investigate the effects from the even earlier Universe, reheating era. We find that in the usual case with T-RH >> m(DM), the production rate during reheating is very small as long as the effective operators dimension d <= 8. Besides, we consider the contribution from the mediator, which may be produced during reheating. Moreover, we study the situation when T-RH is even lower than m(DM) and DM can be directly produced during reheating if its mass does not exceed T-MAX.