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

We investigate the effects of matter-enhanced (MSW) transformation of neutrinos in the Early Universe on the primordial helium yield (Yp). We find that Yp is affected much more by the MSWinduced 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 Yp within a causal horizon is −0.005 ≤ δYp ≤ 0.013 for m2νμ,τ −m2νs ≤ 10 eV in the case of νμ,τ -νs-νe mixing, with the lower limit at m 2 νμ,τ −m2νs ≈ 100 eV, and −0.002 ≤ δYp ≤ 0.020 for m2νe −m 2 νs ≤ 1 eV 2 in the case of νe-νs mixing.