Cosmological Dependence of Resonantly Produced Sterile Neutrinos

TL;DR

This paper explores how different pre-BBN cosmologies affect the production and properties of sterile neutrinos, revealing scenarios where they can evade cosmological constraints and exhibit observable features.

Contribution

It systematically analyzes the impact of various pre-BBN cosmologies on resonant sterile neutrino production, identifying conditions for multiple populations and resonant production.

Findings

Sterile neutrinos can have colder and hotter populations in certain cosmologies.

Fully resonant production can occur in a wide parameter space with sufficient lepton asymmetry.

Sterile neutrinos in the eV-mass range can evade constraints under specific pre-BBN conditions.

Abstract

The detection of a sterile neutrino could constitute the first observation of a particle that could have been produced before Big-Bang Nucleosynthesis (BBN), and could provide information about the yet untested pre-BBN era. The cosmological evolution in this era could be drastically different than typically assumed in what constitutes the standard cosmology, as happens in a variety of motivated particle models. In this work we assess the sensitivity to different pre-BBN cosmologies in which entropy is conserved of 0.01 eV to 1 MeV mass sterile neutrinos produced in the early Universe via resonant active-sterile oscillations, which requires a large lepton asymmetry. We identify mass ranges where it is possible to have two populations of the same sterile neutrino, one with a colder and one with a hotter momentum spectra, which is in principle an observable effect. Furthermore, we show the regions in mass and mixing where fully resonant production (i.e. simultaneously coherent and adiabatic) can occur. We find that in several of the cosmologies we consider, including the standard one, for a lepton asymmetry larger than $\sim10^{-4}$ fully resonantly produced sterile neutrinos in the eV-mass range can evade all cosmological constraints.