Neutrino degeneracy effect on neutrino oscillations and primordial   helium yield
  In this work we present the results of our numerical analysis of the effect of lepton asymmetries L on neutrino oscillations and consequently on helium production in a model of cosmological nucleosynthesis with active-sterile neutrino oscillations. We performed detail calculations of the neutron-proton ratio till its freezing and the subsequent synthesis of helium-4 for a wide range of values of the initial lepton asymmetry 10^{-4}-10^{-10} and for the complete set of mixing parameters of the oscillation model described in hep-ph/9707282: \delta m^2 \le 10^{-7} eV^2 and any \vartheta.   We have found that there are significant modifications of neutrino number densities and energy distributions over a large range of values of the initial asymmetry. Hence, the previously derived (for the case of lepton asymmetry of the order of the baryon one) nucleosynthesis limits on the oscillation parameters are considerably altered. We have calculated the dependence of primordially produced helium-4 on the parameters of the model Y_p(L, \delta m^2, \vartheta). From the isohelium contours in the \delta m^2-\vartheta plane, we have obtained limits on the neutrino mixing parameters corresponding to nucleosynthesis with degenerate neutrinos. An intriguing new result of our analysis is that the lepton asymmetry is able to tighten the nucleosynthesis bounds on neutrino mixing parameters, besides its well known ability to relax them. 
Daniela Kirilova (ICTP) and Mihail Chizhov (ICTP)