Relic neutrino decoupling including flavour oscillations

Abstract

In the early universe, neutrinos are slightly coupled when electron–positron pairs annihilate transferring their entropy to photons. This process originates non-thermal distortions on the neutrino spectra which depend on neutrino flavour, larger for ν e than for ν μ or ν τ . We study the effect of three-neutrino flavour oscillations on the process of neutrino decoupling by solving the momentum-dependent kinetic equations for the neutrino spectra. We find that oscillations do not essentially modify the total change in the neutrino energy density, giving N eff = 3.046 in terms of the effective number of neutrinos, while the small effect over the production of primordial 4He is increased by O ( 20 % ) , up to 2.1 × 10 −4 . These results are stable within the presently favoured region of neutrino mixing parameters.