Active-sterile neutrino oscillations in the early Universe: Asymmetry generation at low vertical bar delta m(2)vertical bar and the Landau-Zener approximation

Abstract

It is well established that active-sterile neutrino oscillations generate large neutrino asymmetries for very small mixing angles (sin22θ0≲10-4), negative values of δm2, provided that |δm2|≳10-4 eV2. By numerically solving the quantum kinetic equations, we show that the generation still occurs at much lower values of |δm2|. 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..