On Lepton asymmetry and BBN

The strongest available cosmological constraint on lepton asymmetry is L<0.01. We discuss in more detail a BBN model with late ν_e↔ν_s oscillations which is capable of measuring extremely small lepton asymmetry, L>10⁻⁸. This sensitivity is achieved through the influence of small L on the neutrino oscillations, suppressing or enhancing them, and thus decreasing or increasing the primordially produced ⁴He. The cases of asymmetry generated by late resonant electron-sterile oscillations and relic lepton asymmetry are considered. The influence of L on nucleons freezing in pre-BBN epoch is numerically analyzed in the full range of the oscillation parameters for the model and L≥10⁻¹⁰.