Multi-momentum and multi-flavour active-sterile neutrino oscillations in the early universe: role of neutrino asymmetries and effects on nucleosynthesis

TL;DR

This study investigates how multi-flavour, multi-momentum neutrino oscillations in the early universe, influenced by neutrino asymmetries, affect sterile neutrino production and primordial nucleosynthesis, revealing significant deviations from simplified models.

Contribution

It introduces a detailed multi-momentum, multi-flavour kinetic treatment of neutrino oscillations, showing larger asymmetries are needed to suppress sterile neutrino production and highlighting their impact on nucleosynthesis.

Findings

Enhanced sterile neutrino production compared to average momentum approximation.

Large neutrino asymmetries delay active-sterile conversions, causing spectral distortions.

Increased helium abundance due to spectral distortions and asymmetries.

Abstract

We perform a study of the flavour evolution in the early universe of a multi-flavour active-sterile neutrino system with parameters inspired by the short-baseline neutrino anomalies. In a neutrino-symmetric bath a "thermal" population of the sterile state would quickly grow, but in the presence of primordial neutrino asymmetries a self-suppression as well as a resonant sterile neutrino production can take place, depending on temperature and chosen parameters. In order to characterize these effects, we go beyond the usual average momentum and single mixing approximations and consider a multi-momentum and multi-flavour treatment of the kinetic equations. We find that the enhancement obtained in this case with respect to the average momentum approximation is significant, up to \sim 20 % of a degree of freedom. Such detailed and computationally demanding treatment further raises the asymmetry values required to significantly suppress the sterile neutrino production, up to large and preferentially net asymmetries |L_{\nu}| > O(10^{-2}). For such asymmetries, however, the active-sterile flavour conversions happen so late that significant distortions are produced in the electron (anti)neutrino spectra. The larger |L_{\nu}|, the more the impact of these distortions takes over as dominant cosmological effect, notably increasing the 4 He abundance in primordial nucleosynthesis (BBN). The standard expression of the primordial yields in terms of the effective number of neutrinos and asymmetries is also greatly altered. We numerically estimate the magnitude of such effects for a few representative cases and comment on possible implications for forthcoming cosmological measurements.