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 ${L}_{{\ensuremath{\nu}}_{e}}$ by ${\ensuremath{\nu}}_{e}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{s}$ oscillations. In the second case, we consider the mass hierarchy ${m}_{{\ensuremath{\nu}}_{\ensuremath{\tau}}}\ensuremath{\gg}{m}_{{\ensuremath{\nu}}_{\ensuremath{\mu}}}{,m}_{{\ensuremath{\nu}}_{e}}{,m}_{{\ensuremath{\nu}}_{s}}$ where ${\ensuremath{\nu}}_{\ensuremath{\tau}}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{s}$ oscillations generate a large ${L}_{{\ensuremath{\nu}}_{\ensuremath{\tau}}}$ and some of this asymmetry is converted into ${L}_{{\ensuremath{\nu}}_{e}}$ by ${\ensuremath{\nu}}_{\ensuremath{\tau}}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{e}$ oscillations. We estimate the implications for BBN for a range of cosmologically interesting $\ensuremath{\delta}{m}^{2}$ values. The present paper improves on a previous study 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.