Prethermalization production of dark matter

At the end of inflation, the inflaton field decays into an initially nonthermal population of relativistic particles which eventually thermalize. We consider the production of dark matter via freeze-in from this relativistic plasma, focusing on the prethermal phase. We find that for a production cross section sigma(E) similar to E-n with n > 2, the present dark matter abundance is produced during the prethermal phase of its progenitors. For n <= 2, entropy production during reheating makes the nonthermal contribution to the present dark matter abundance subdominant compared to that produced thermally. As specific examples, we verify that the nonthermal contribution is irrelevant for gravitino production in low scale supersymmetric models (n = 0) and is dominant for gravitino production in high scale supersymmetry models (n = 6).