Lepton asymmetry effect on neutrino oscillations and primordial [sup 4]He

We analyze the effects of lepton asymmetry on neutrino oscillations and oncosmological nucleosynthesis with active-sterile oscillating neutrinos. It isshown that small lepton asymmetries, L < 0.01, whose direct kinetic effect onnucleosynthesis is negligible, still effect nucleosynthesis considerablythrough their influence on oscillating neutrinos. Two different cases of leptonasymmetry are discussed: an initially present and a dynamically generated inoscillations. Dynamically generated in resonant oscillations asymmetry at smallmixing angles suppresses oscillations, hence, the nucleosynthesis bounds onneutrino mass differences at small mixings are relaxed. Initially presentasymmetry may suppress or enhance oscillations. The enhancement is a result ofinterchanging resonances between neutrino and antineutrino ensembles due toresonance waves passing through the neutrino and antineutrino spectrum. Updatednucleosynthesis bounds on neutrino oscillation parameters accounting for leptonasymmetry are presented.Comment: Extended version of the talk presented by D. Kirilova at the conference Cosmology and Astroparticle Physics (CAPP 2000), Verbier, Switzerland (July 17-28, 2000). To appear in ``Cosmology and Astroparticle Physics'', Proc. of CAPP 2000, eds. J. Garcia-Bellido, R. Durrer and M. Shaposhnikov (AIP, 2001