Relic density of neutrinos with primordial asymmetries

It is well known that flavor oscillations in the early universe roughly equalize possible primordial neutrino-antineutrino asymmetries, so the restrictive BBN bound on the nu_e anti-nu_e asymmetry applies to all flavors. However, oscillations driven by Delta m^2_sol begin only when neutrino interactions freeze out (temperature of a few MeV) and equilibrated densities and spectra are not guaranteed. Including for the first time collisions and pair processes in the kinetic equations for mixed neutrinos we find approximate equilibrium only if the mixing angle theta_13 is not too small. Otherwise an excess radiation density can survive that may show up in future cosmological precision data