Cosmological abundances of right-handed neutrinos
  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. 
Kari Enqvist, Petteri Ker\"anen, Jukka Maalampi and Hannes Uibo   (Department of Physics, University of Helsinki, Finland)