Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology

We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter xi common to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance Y-p is the most sensitive cosmic leptometer. Adopting the conservative Y-p error analysis of Olive and Skillman implies -0.04 less than or similar to xi less than or similar to 0.07 whereas the errors stated by Izotov and Thuan imply xi=0.0245 +/- 0.0092 (1 sigma). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of Y-p that reliably distinguishes between a vanishing or nonvanishing xi is a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.