Large neutrino asymmetry from forbidden decay of dark matter

TL;DR

This paper explores how early Universe dark matter decay, triggered by a first order phase transition, can generate neutrino asymmetry and gravitational waves, with potential observable signals in upcoming experiments.

Contribution

It introduces a mechanism linking dark matter decay during a phase transition to neutrino asymmetry and gravitational wave production, with testable predictions.

Findings

Dark matter decay can produce large neutrino asymmetry affecting cosmological parameters.

The phase transition can generate gravitational waves detectable by future observatories.

The model explains the Helium anomaly and predicts signals within experimental reach.

Abstract

Dark matter (DM), in spite of being stable or long-lived on cosmological scales, can decay in the early Universe due to finite-temperature effects. In particular, a first order phase transition (FOPT) in the early Universe can provide a finite window for such decay, guaranteeing DM stability at lower temperatures, consistent with observations. The FOPT can lead to the generation of stochastic gravitational waves (GW) with peak frequencies correlated with DM mass. On the other hand, early DM decay into neutrinos can create a large neutrino asymmetry which can have interesting cosmological consequences in terms of enhanced effective relativistic degrees of freedom $N_{\rm eff}$, providing a solution to the recently observed Helium anomaly among others. Allowing DM decay to occur below sphaleron decoupling temperature, thereby avoiding overproduction of baryon asymmetry, forces the FOPT to occur at sub-electroweak scale. This leaves the stochastic GW within range of experiments like LISA, $\mu$ARES, NANOGrav etc.