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Neutrino-mixing-generated lepton asymmetry and the primordial 4He abundance


It has been proposed that an asymmetry in the electron neutrino sector may be generated by resonant active-sterile neutrino transformations during big bang nucleosynthesis (BBN). We calculate the change in the primordial 4He yield Y resulting from this asymmetry, taking into account both the time evolution of the νe and ν̄e distribution function and the spectral distortions in these. We calculate this change in two schemes: (1) a lepton asymmetry directly generated by νe mixing with a lighter right-handed sterile neutrino νs, and (2) a lepton asymmetry generated by a ντ↔νs or νμ↔νs transformation which is subsequently partially converted to an asymmetry in the νeν̄e sector by a matter-enhanced active-active neutrino transformation. In the first scheme, we find that the percentage change in Y is between -1% and 9% (with the sign depending on the sign of the asymmetry), bounded by the Majorana mass limit mνe≲1 eV. In the second scheme, the maximal percentage reduction in Y is 2%, if the lepton number asymmetry in neutrinos is positive; otherwise, the percentage increase in Y is ≲5% for mνμ,ντ2-mνs2≲104 eV. We conclude that the change in the primordial 4He yield induced by a neutrino-mixing-generated lepton number asymmetry can be substantial in the upward direction, but limited in the downward direction. ©1999 The American Physical Society.

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