Active-Sterile Neutrino Mixing in the Early Universe and Primordial Nucleosynthesis

Abstract

We investigate the effects of matter-enhanced (MSW) transformation of neutrinos in the Early Universe on the primordial helium yield (Y_p). We find that Y_p is affected much more by the MSW-induced alterations in the neutrino energy spectra than by the associated change in expansion rate. Specifically, the absence due to transformation of low energy electron neutrinos can significantly affect neutron-proton weak interconversion rates through the lifting of Fermi-blocking of neutron decay at low energies and through halting low-energy neutrino capture on neutrons. We find that the change of Y_p within a causal horizon is -0.005\le δY_p \leq 0.013 for m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \le 10^4 eV^2 in the case of ν_{μ,τ}-ν_s-ν_e mixing, with the lower limit at m_{ν_{μ,τ}}^2 - m_{ν_s}^2 \approx 100 eV^2, and -0.002\le δY_p \leq 0.020 for m_{ν_e}^2 - m_{ν_s}^2 \le 1 eV^2 in the case of ν_e-ν_s mixing.