Freeze-in Dark Matter from Secret Neutrino Interactions
  We investigate a simplified freeze-in dark-matter model in which the dark
matter only interacts with the standard-model neutrinos via a light scalar. The
extremely small coupling for the freeze-in mechanism is naturally realized in
several neutrino-portal scenarios with the secret neutrino interactions. We
study possible evolution history of the dark sector: the dark sector would
undergo pure freeze-in production if the interactions between the dark-sector
particles are negligible, while thermal equilibrium within the dark sector
could occur if the reannihilation of the dark matter and the scalar mediator is
rapid enough. We investigate the relic abundance in the freeze-in and dark
freeze-out regimes, calculate evolution of the dark temperature, and study its
phenomenological aspects on BBN and CMB constraints, the indirect-detection
signature, as well as the potential to solve the small scale structure problem.
