Detailed study of BBN implications of neutrino oscillation generated neutrino asymmetries in some four neutrino models

Abstract

We re-examine the evolution of neutrino asymmetries in several four neutrino models. The first case involves the direct creation of $L_{ν_e}$ by $ν_e \leftrightarrow ν_s$ oscillations. In the second case, we consider the mass hierarchy $m_{ν_τ} \gg m_{ν_μ}, m_{ν_e}, m_{ν_s}$ where $ν_τ\leftrightarrow ν_s$ oscillations generate a large $L_{ν_τ}$ and some of this asymmetry is converted into $L_{ν_e}$ by $ν_τ \leftrightarrow ν_{e}$ oscillations. We estimate the implications for BBN for a range of cosmologically interesting $δm^2$ values. The present paper improves on previous published work by taking into account the finite repopulation rate and the time dependence of the distortions to the neutrino momentum distributions. The treatment of chemical decoupling is also improved.