Boosting Ultraviolet Freeze-in in NO Models
  We present a novel UV freeze-in mechanism for enhancing the dark matter
abundance in cosmologies where inflation is followed by an epoch dominated by a
fluid stiffer than radiation. In such scenarios, even a small radiation
abundance, produced for instance by instantaneous preheating effects, will
eventually dominate the total energy density of the Universe without the need
for a complete inflaton decay. For the sake of concreteness, we focus in
non-oscillatory (NO) quintessential inflation models, albeit our treatment is
rather general and can be extended to other scenarios. The high temperature of
the initial thermal bath together with the absence of subsequent entropy
injections into the Standard Model plasma translates into a highly-effective UV
DM freeze-in. In particular, we find that, during kination, an enhancement in
the DM abundance is generically obtained for any production cross-section
not-decreasing with temperature.
