Playing with lepton asymmetry at the resonant production of sterile neutrino dark matter

TL;DR

This paper investigates how lepton asymmetry in the early universe affects sterile neutrino dark matter production, potentially relaxing observational constraints and opening new parameter space for lighter sterile neutrinos detectable by future X-ray telescopes.

Contribution

It demonstrates that specific flavor asymmetries can cancel active neutrino oscillation effects, reducing constraints on sterile neutrino properties and enabling colder spectra, thus expanding viable dark matter models.

Findings

Lepton flavor asymmetry can mitigate BBN constraints on sterile neutrinos.

Disappearance of lepton asymmetry leads to colder neutrino spectra.

New parameter space for lighter sterile neutrinos is opened for future detection.

Abstract

We examine the sterile neutrino dark matter production in the primordial plasma with lepton asymmetry unequally distributed over different neutrino flavors. We argue that with the specific flavor fractions, one can mitigate limits from the Big Bang Nucleosynthesis on the sterile-active neutrino mixing angle and sterile neutrino mass. It happens due to cancellation of the neutrino flavor asymmetries in active neutrino oscillations, which is more efficient in the case of inverse hierarchy of active neutrino masses and does not depend on the value of CP-phase. This finding opens a window of lower sterile-active mixing angles. Likewise, we show that, with lepton asymmetry disappearing from the plasma at certain intermediate stages of the sterile neutrino production, the spectrum of produced neutrinos becomes much colder, which weakens the limits on the model parameter space from observations of cosmic small-scale structures (Ly-$\alpha$ forest, galaxy counts, etc.). This finding reopens the region of lighter sterile neutrinos. The new region may be explored with the next generation of X-ray telescopes searching for the inherent peak signature provided by the dark matter sterile neutrino radiative decays in the Galaxy.