Lepton asymmetry creation in the early universe

Oscillations of active to sterile neutrinos with a small mixing angle sin2θ<10−2 could generate a large lepton asymmetry in the early universe. The final order of magnitude of the lepton asymmetry η is mainly determined by its growth in the last stage of evolution, the so-called power-law regime. There exist two contradictory results in the literature, η∝T−1 and T−4, where T is the background medium temperature. In the first case, the lepton asymmetry does not exceed values of 10−4 for |δm2|⩽1 eV2, while in the second case it can become larger than 10−1. In this work we analytically investigate the case η∝T−1, using a new approach to solve the kinetic equations. We find that the power-law solution η∝T−1 is not self-consistent. Instead, we find the power law η∝T−11/3 to be a good approximation, which leads to a large final asymmetry.