How Much Lepton Asymmetry Can the Neutrino Seas Hold?

Lepton number need not reside in baryons or charged leptons; it can be stored as a chemical potential in the cosmic neutrino seas, where it is nearly invisible. This paper computes the storage capacity and its observable side effects. A degeneracy parameter xi = mu/T of 0.1 in each flavour stores a lepton yield per flavour of 1.45e-02, vastly more than the baryon yield, while shifting the effective number of relativistic species by only 0.013 and the helium mass fraction to 0.226 if the electron flavour shares the asymmetry. At xi = 0.01 the stored yield is 1.45e-03 and the shift in relativistic species is negligible; at xi = 0.5 the shift reaches 0.330, within reach of current limits. Storing the entire observed asymmetry equivalent in neutrinos would need only xi of order 5.98e-10. A bounce history passes the neutrino seas through decoupling twice, but storage established after the final decoupling is what observations constrain. Computed from stated assumptions, not measured.