Transformation-Induced Nonthermal Neutrino Spectra and Primordial Nucleosynthesis

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We examine in detail the changes in the production of primordial helium resulting from nonthermal neutrino momentum distributions produced by resonant transformation of electron-type neutrinos to steriles. These transformations, anti-nuₑ -> anti-nuₛ (nuₑ->nuₛ), amplify a positive (negative) lepton number asymmetry. We find that the resulting supression relative to a thermal distribution of low energy nuₑ reduces n -> p conversion to a greater extent than does the enhancement of n -> p from an identical suppression of anti-nuₑ. Thus, equal lepton-number asymmetries of opposite sign have unequal effects on the resulting helium yield in primordial nucleosynthesis.