Light Dark Matter from Entropy Dilution

TL;DR

This paper proposes a scenario where a relativistic thermal relic can serve as dark matter if entropy dilution occurs after decoupling, allowing very light dark matter candidates without conflicting with cosmological constraints.

Contribution

It introduces a mechanism of entropy dilution via heavy state decays to make hot relics viable dark matter candidates, with detailed model constraints and experimental prospects.

Findings

Diluted hot relics can be as light as a keV and still account for all dark matter.

The entropy dilution mechanism is achieved through decays of a heavy state in the early universe.

Constraints from terrestrial experiments, astrophysics, and cosmology are analyzed for the proposed model.

Abstract

We show that a thermal relic which decouples from the standard model (SM) plasma while relativistic can be a viable dark matter (DM) candidate, if the decoupling is followed by a period of entropy dilution that heats up the SM, but not the dark sector. Such diluted hot relics can be as light as a keV, while accounting for the entirety of the DM, and not conflicting with cosmological and astrophysical measurements. The requisite dilution can be achieved via decays of a heavy state that dominates the energy budget of the universe in the early matter dominated era. The heavy state decays into the SM particles, heats up the SM plasma, and dilutes the hidden sector. The interaction required to equilibrate the two sectors in the early universe places a bound on the maximum possible dilution as a function of the decoupling temperature. As an example of diluted hot relic DM we consider a light Dirac fermion with a heavy dark photon mediator. We present constraints on the model from terrestrial experiments (current and future), astrophysics, and cosmology.