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

Original · EN

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 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 Yₚ within a causal horizon is -0.005≤ δYₚ ≤ 0.013 for mνμ,τ² - mνₛ² ≤ 10⁴ eV² in the case of νμ,τ-νₛ-νₑ mixing, with the lower limit at mνμ,τ² - mνₛ² ≈ 100 eV², and -0.002≤ δYₚ ≤ 0.020 for mνₑ² - mνₛ² ≤ 1 eV² in the case of νₑ-νₛ mixing.