  We study the equilibration of the right-helicity states of light Dirac
neutrinos in the early universe by solving the momentum dependent Boltzmann
equations numerically. We show that the main effect is due to electroweak gauge
boson poles, which enhance thermalization rates by some three orders of
magnitude. The right-helicity states of tau neutrinos will be brought in
equilibrium independently of their initial distribution at a temperature above
the poles if the tau neutrino mass is larger than about 10 keV.
