Neutrino Portal to FIMP Dark Matter with an Early Matter Era
  We study the freeze-in production of Feebly Interacting Massive Particle
(FIMP) dark matter candidates through a neutrino portal. We consider a hidden
sector comprised of a fermion and a complex scalar, with the lightest one
regarded as a FIMP candidate. We implement the Type-I Seesaw mechanism for
generating the masses of the Standard Model (SM) neutrinos and consider three
heavy neutrinos, responsible for mediating the interactions between the hidden
and the SM sectors. We assume that an early matter-dominated era (EMDE) took
place for some period between inflation and Big Bang Nucleosynthesis, making
the Universe to expand faster than in the standard radiation-dominated era. In
this case, the hidden and SM sectors are easily decoupled and larger couplings
between FIMPs and SM particles are needed from the relic density constraints.
In this context, we discuss the dynamics of dark matter throughout the modified
cosmic history, evaluate the relevant constraints of the model and discuss the
consequences of the duration of the EMDE for the dark matter production.
Finally, we show that if the heavy neutrinos are not part of the thermal bath,
this scenario becomes testable through indirect detection searches.
