Primordially produced helium-4 in the presence of neutrino oscillations

The production of helium-4 during the cosmological nucleosynthesis in the presence of active-sterile neutrino oscillations, ve↔vs, efficient after decoupling of electron neutrino, is analyzed. All known oscillation effects on primordial nucleosynthesis, namely: increase of the effective degrees of freedom during nucleosynthesis, neutrino spectrum distortion, depletion of electron neutrino number density and generation of neutrino-antineutrino asymmetry, are precisely taken into account. Primordially produced 4He abundance is calculated, in a self-consistent study of the kinetics of the nucleons and the oscillating neutrinos, for the full range of parameters of the oscillation model with small mass differences: δm2≤10-7 eV2. A considerable relative increase of helium-4, up to 14% for non-resonant oscillations and up to 32% for resonant ones is registered for a certain interval of oscillations parameters values. Combined isohelium contours δYp/Yp=3%,5%,7% for resonant and non-resonant oscillations are presented. Cosmological constraints on oscillation parameters and on the sterile solar neutrino solutions are discussed