Light sterile neutrino production in the early universe with dynamical neutrino asymmetries

TL;DR

This paper studies how dynamical neutrino asymmetries and CP-violation affect the production of light sterile neutrinos in the early universe, with implications for cosmological observations and existing anomalies.

Contribution

It provides the first detailed analysis of sterile neutrino production considering evolving asymmetries and CP-violation effects in (3+1) and (2+1) schemes.

Findings

Equal active neutrino asymmetries of |L|=10^{-3} suppress resonant sterile production.

Opposite asymmetries increase sterile abundance, requiring larger |L|≈10^{-2} for suppression.

CP-violation enhances sterile neutrino production and spectral distortions.

Abstract

Light sterile neutrinos mixing with the active ones have been recently proposed to solve different anomalies observed in short-baseline oscillation experiments. These neutrinos can also be produced by oscillations of the active neutrinos in the early universe, leaving possible traces on different cosmological observables. Here we perform an updated study of the neutrino kinetic equations in (3+1) and (2+1) oscillation schemes, dynamically evolving primordial asymmetries of active neutrinos and taking into account for the first time CP-violation effects. In the absence of neutrino asymmetries, eV-mass scale sterile neutrinos would be completely thermalized creating a tension with respect to the CMB, LSS and BBN data. In the past literature, active neutrino asymmetries have been invoked as a way to inhibit the sterile neutrino production via the in-medium suppression of the sterile-active mixing angle. However, neutrino asymmetries also permit a resonant sterile neutrino production. We find that if the active species have equal asymmetries L, a value |L|=10^{-3} is required to start suppressing the resonant sterile production, roughly an order of magnitude larger than what previously expected. When active species have opposite asymmetries the sterile abundance is further enhanced, requiring an even larger |L|\simeq 10^{-2} to start suppressing their production. In the latter case, CP-violation (naturally expected) further exacerbates the phenomenon. Some consequences for cosmological observables are briefly discussed: for example, it is likely that moderate suppressions of the sterile species production are associated with significant spectral distortions of the active neutrino species, with potentially interesting phenomenological consequences especially for BBN.