Nonequilibrium neutrino oscillations and primordial production of 4He

We studied nonequilibrium oscillations between left-handed electron neutrinos and nonthermalized sterile neutrinos in a medium for resonant and nonresonant cases. In our work the exact kinetic equations for neutrinos written in terms of neutrino density matrix in momentum space were analyzed. The evolution of the neutrino density matrix was numerically calculated. This allowed us to study precisely the evolution of the neutrino number densities, energy spectrum distortion and the asymmetry between neutrinos and antineutrinos for each momentum mode. Cosmological constraints on the mixing parameters were obtained on the basis of a detail numerical study of the influence of nonequilibrium oscillations on the primordial production of 4He.