A Thermal Neutrino Portal to Sub-MeV Dark Matter
  Thermal relics lighter than an MeV contribute to the energy density of the
universe at the time of nucleosynthesis and recombination. Constraints on extra
radiation degrees of freedom typically exclude even the simplest of such dark
sectors. We explore the possibility that a sub-MeV dark sector entered
equilibrium with the Standard Model after neutrino-photon decoupling, which
significantly weakens these constraints and naturally arises in the context of
neutrino mass generation through the spontaneous breaking of lepton number.
Acquiring an adequate dark matter abundance independently motivates the
MeV-scale in these models through the coincidence of gravitational,
matter-radiation equality, and neutrino mass scales, $(m_\text{Pl} /
T^\text{MRE})^{1/4} \, m_\nu \sim \text{MeV}$. This class of scenarios will be
decisively tested by future measurements of the cosmic microwave background and
matter structure of the universe. While the dark sector dominantly interacts
with Standard Model neutrinos, large couplings to nucleons are possible in
principle, leading to observable signals at proposed low-threshold direct
detection experiments.
