  We study dark matter freeze-in scenarios where the mass of the mediator
particle that couples dark matter to the Standard Model is larger than the
reheat temperature, TRH, in the early Universe. In such setups, the standard
approach is to work with an effective field theory (EFT) where the mediator is
integrated out. We examine the validity of this approach in various generic s-
and t-channel mediator frameworks. We find that the EFT approach breaks down
when the mediator mass is between one to two orders of magnitude larger than
TRH due to various effects such as s-channel resonance, a small
thermally-suppressed abundance of the mediator, or decays of Standard Model
particles through loops induced by the mediator. This highlights the necessity
of including these contributions in such dark matter freeze-in studies. We also
discuss the collider phenomenology of the heavy mediators, which is
qualitatively different from standard freeze-in scenarios. We highlight that,
due to the low TRH, the Standard Model-dark matter coupling in these scenarios
can be relatively larger than in standard freeze-in scenarios, improving the
testability prospects of these setups.
