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Author: Kevork Abazajian, Xiangdong Shi, George M. Fuller (UCSD)

Title: Active-Sterile Neutrino Mixing in the Early Universe and PrimordialNucleosynthesis

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 \delta Y_p \leq 0.013 for m_{\nu_{\mu,\tau}}^2 - m_{\nu_s}^2 \le 10^4 eV^2 in the case of \nu_{\mu,\tau}-\nu_s-\nu_e mixing, with the lower limit at m_{\nu_{\mu,\tau}}^2 - m_{\nu_s}^2 \approx 100 eV^2, and -0.002\le \delta Y_p \leq 0.020 for m_{\nu_e}^2 - m_{\nu_s}^2 \le 1 eV^2 in the case of \nu_e-\nu_s mixing.

Comments: 6 pages, Latex, proceedings for Division of Particles and Fields Conference, University of California, Los Angeles,January 5-9, 1999


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