A Thermal Neutrino Portal to Sub-MeV Dark Matter

TL;DR

This paper proposes a sub-MeV dark sector that entered equilibrium after neutrino-photon decoupling, weakening cosmological constraints and linking dark matter, neutrino masses, and cosmic scales, with testable predictions for future experiments.

Contribution

It introduces a new class of dark sector models that naturally explain MeV-scale dark matter and neutrino masses, with distinctive cosmological and experimental signatures.

Findings

Sub-MeV dark sector weakens cosmological constraints.

Models link dark matter, neutrino masses, and cosmic scales.

Potential signals in future CMB, matter structure, and direct detection experiments.

Abstract

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.