Sterile neutrinos produced near the EW scale I: mixing angles, MSW resonances and production rates

TL;DR

This paper investigates sterile neutrino production near the electroweak scale, analyzing mixing, resonances, and rates, revealing non-thermal distributions with implications for cosmology.

Contribution

It provides a detailed calculation of sterile neutrino production mechanisms, including MSW resonances and damping effects, in a beyond-standard-model framework at electroweak energies.

Findings

Identification of narrow MSW resonances without lepton asymmetry.

Discovery of strong damping near resonances affecting mode degeneracy.

Prediction of non-thermal sterile neutrino distributions impacting small-scale structure.

Abstract

We study the production of sterile neutrinos in the region $T\sim M_W$ in an extension beyond the standard model with the see-saw mass matrix originating in Yukawa couplings to Higgs-like scalars with masses and vev's of the order of the electroweak scale. Sterile neutrinos are produced by the decay of scalars and standard model vector bosons. We obtain the index of refraction, dispersion relations, mixing angles in the medium and production rates including those for right-handed sterile neutrinos, from the standard model and beyond the standard model self-energies. For $1 \lesssim M_W/T \lesssim 3$ we find narrow MSW resonances with $k \lesssim T$ for both left and right handed neutrinos even in absence of a lepton asymmetry in the (active) neutrino sector, as well as very low energy ($k/T \ll |\xi|$) narrow MSW resonances in the presence of a lepton asymmetry consistent with the bounds from WMAP and BBN. For small vacuum mixing angle, consistent with observational bounds, the absorptive part of the self-energies lead to a strong damping regime very near the resonances resulting in the \emph{exact} degeneracy of the propagating modes with a concomitant breakdown of adiabaticity. We argue that cosmological expansion sweeps through the resonances, \emph{resonant and non-resonant} sterile neutrino production results in a highly \emph{non-thermal} distribution function enhanced at small momentum $k < T$, with potentially important consequences for their free streaming length and transfer function at small scales.