Bounds on Dirac neutrino masses from nucleosynthesis

We derive new bounds on the Dirac mass of the tau and muonic neutrinos. By solving the kinetic equation for the rate of energy deposition due to helicity-flipping processes and imposing the constraint that the number of effective species contributing to the energy density at the time of nucleosynthesis be Δ${\mathit{k}}_{\nu }$${\mathit{m}}_{\nu \mathrm{\mu }}$${\mathit{m}}_{\nu \mathrm{\tau }}$${\mathit{T}}_{\mathrm{QCD}}$=200 MeV. The constraint Δ${\mathit{k}}_{\nu }$${\mathit{m}}_{\nu }$