Lower Mass Bounds on FIMP Dark Matter Produced via Freeze-In
  Feebly Interacting Massive Particles (FIMPs) are dark matter candidates that
never thermalize in the early universe and whose production takes place via
decays and/or scatterings of thermal bath particles. If FIMPs interactions with
the thermal bath are renormalizable, a scenario which is known as freeze-in,
production is most efficient at temperatures around the mass of the bath
particles and insensitive to unknown physics at high temperatures. Working in a
model-independent fashion, we consider three different production mechanisms:
two-body decays, three-body decays, and binary collisions. We compute the FIMP
phase space distribution and matter power spectrum, and we investigate the
suppression of cosmological structures at small scales. Our results are lower
bounds on the FIMP mass. Finally, we study how to relax these constraints in
scenarios where FIMPs provide a sub-dominant dark matter component.
