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Cosmological Nucleosynthesis and Active-Sterile Neutrino Oscillations with Small Mass Differences: The Nonresonant Case

We study the nonresonant oscillations between left-handed electron neutrinos nu_e and nonthermalized sterile neutrinos nu_s in the early Universe plasma. The case when nu_s do not thermalize till 2 MeV and the oscillations become effective after nu_e decoupling is discussed. The evolution of neutrino ensembles was described numerically by integrating the kinetic equations for the neutrino density matrix in momentum space. We have provided a detail numerical study of the influence of the nonequilibrium nu_e - nu_s oscillations on the primordial production of He-4. The exact kinetic approach enabled us to calculate the effects of neutrino population depletion, the distortion of the neutrino energy spectrum and the generation of neutrino-antineutrino asymmetry on the kinetics of neutron-to-proton transitions during the primordial nucleosynthesis epoch and correspondingly on the cosmological He-4 production. It was shown that the neutrino population depletion and spectrum distortion play an important role. The asymmetry effect, in case the lepton asymmetry is accepted initially equal to the baryon one, is proved to be negligible for the discussed range of delta m^2. Constant helium contours in delta m^2 - theta plain were obtained for small mass differences delta m^2 < 10^{-7} eV^2. More precise cosmological constraints on mixing parameters were obtained.