Nonequilibium Neutrino Oscillations and Primordial Nucleosynthesis

Abstract

We studied nonequilibrium oscillations between left-handed electron neutrinos and nonthermalized sterile neutrinos. The exact kinetic equations for neutrinos, written in terms of neutrino density matrix in momentum space were analyzed. The evolution of neutrino density matrix was numerically calculated. This allowed to study precisely the evolution of the neutrino number densities, energy spectrum distortion and the asymmetry between neutrinos and antineutrinos for each momentum mode. Both effects of distortion and asymmetry, which cannot be accounted for correctly when working in terms of particle densities and mean energies, and the depletion of electron neutrino state have been proved considerable for a certain range of oscillation parameters. The influence of nonequilibrium oscillations on primordial nucleosynthesis was calculated. Cosmologically excluded regions for oscillation parameters were obtained.