UC San Diego

We discuss big bang nucleosynthesis constraints on maximal νμ↔νs mixing. Vacuum νμ↔νs oscillation has been proposed as one possible explanation of the Super Kamiokande atmospheric neutrino data. Based on the most recent primordial abundance measurements, we find that the effective number of neutrino species for big bang nucleosynthesis (BBN) is Nv≲3.3. Assuming that all three active neutrinos are light (with masses ≪; 1 MeV), we examine BBN constraints on νμ↔νs mixing in two scenarios: (1) a negligible lepton asymmetry (the standard picture) and (2) the presence of a large lepton asymmetry which has resulted from an amplification by ντ↔νs′, mixing (νs′ being νs or another sterile neutrino species). The latter scenario has been proposed recently to reconcile the BBN constraints and large-angle νμ↔νs mixing. We find that the large-angle νμ↔νs mixing in the first scenario, which would yield Nv≈4, is ruled out as an explanation of the Super Kamiokande data. It is conceivably possible for the νμ↔νs solution to evade BBN bounds in the second scenario, but only if 200eV2≲mντ2-m νs′2≲104 eV2is satisfied, and if ντ decays non-radiatively with a lifetime ≲103 yr. This mass-squared difference implies 15eV≲mντ ≲100 eV if νs is much lighter than ντ. We conclude that maximal (or near maximal) νμ↔ντ mixing is a more likely explanation of the Super Kamiokande data. ©1999 The American Physical Society.