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We use heavy element nucleosynthesis from supernovae to probe the mixing of ₄ with _ (or _) possessing cosmologically significant masses (1 to 100 eV). We conclude that the _ (_) -₄ vacuum mixing angle must satisfy sin^2210^-5, in order to ensure that r-process heavy elements can be produced in neutrino-heated supernova ejecta. Mixing at a level exceeding this limit precludes r-process nucleosynthesis in this site.
Qian et al. (Mon,) studied this question.