Light Sterile Neutrinos in the Early Universe: Effects of Altered Dispersion Relations and a coupling to Axion-Like Dark Matter

TL;DR

This paper explores how altered dispersion relations and couplings to axion-like dark matter influence light sterile neutrinos in the early universe, affecting their population and primordial nucleosynthesis outcomes.

Contribution

It introduces a combined analysis of ADRs and scalar couplings on sterile neutrinos, revealing new parameter spaces compatible with cosmological constraints.

Findings

Both effects can suppress sterile neutrino populations individually.

The combined effects expand viable parameter space beyond previous bounds.

Results align with observed helium abundance in nucleosynthesis.

Abstract

We investigate the cosmological consequences of light sterile neutrinos with altered dispersion relations (ADRs) and couplings to an ultra-light, axion-like scalar field. In particular we study the impact on the number of additional, light, fermionic degrees of freedom and primordial nucleosynthesis. While the ADR leads to a new potential term in the Hamiltonian, the coupling to the scalar field results in a time dependent, effective mass contribution. We solve the quantum kinetic equations (QKEs) for the neutrino density matrix and find that in certain parameter regions both new physics effects can individually yield a suppressed population of sterile neutrino species and the correct observed amount of helium in nucleosynthesis. Combining both effects opens up new patches of parameter space excluded by experimental bounds applying to models featuring only one of the effects.