UC San Diego

The implications of the Z0-width experiment for the number of neutrino flavors taken together with the 4He constraint on the energy density at the nucleosynthesis epoch, can give a limit on the extent to which a right-handed neutrino sea could contribute to the expansion rate of the universe during Big Bang nucleosynthesis. Since the population of the right-handed neutrino sea depends on the neutrino mass we can extract a Dirac neutrino mass limit of mv ≪ 150 keV. This limit could apply to νμ or ντ if they have purely Dirac masses. © 1991.