Active-sterile neutrino oscillations in the early Universe: Asymmetry generation at low vertical bar δm2 vertical bar and the Landau-Zener approximation

It is well established that active-sterile neutrino oscillations generate large neutrino asymmetries for very small mixing angles (sin2 2θ0(less-or-similar sign)10-4), negative values of δm2, provided that vertical bar δm2 vertical bar > or approx. 10-4 eV2. By numerically solving the quantum kinetic equations, we show that the generation still occurs at much lower values of vertical bar δm2 vertical bar. We also describe the borders of the generation at small mixing angles and show how our numerical results can be analytically understood within the framework of the Landau-Zener approximation thereby extending previous work based on the adiabatic limit. This approximate approach leads to a fair description of the MSW dominated regime of the neutrino asymmetry evolution and is also able to correctly reproduce its final value. We also briefly discuss the impact that neutrino asymmetry generation could have on big bang nucleosynthesis, CMBR and relic neutrinos