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 $T_{RH}\gg m_{\rm DM}$, the production rate during
reheating is very small as long as the effective operators dimension $d \leq
8$. 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 $m_{\rm DM}$ and DM can be directly produced during reheating
if its mass does not exceed $T_{MAX}$.
