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Previous work has used the primordial abundance of ⁴He to infer limits on the number of neutrinos with full-strength neutral-current weak interactions. By accounting for the quark-gluon constituents of hadrons, we extend the analysis to earlier times and higher temperatures and densities and, therefore, to considerably weaker interactions. The maximum number of new, superweakly interacting, light ({} MeV) particles is between {~}1 and {~}20.
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Steigman et al. (1979) studied this question.
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