Bounds on Dirac Neutrino Masses from Nucleosynthesis

Abstract

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 $Δk_ν<~0.3$, we find the bounds $m_{ν_μ} < ~150$ KeV and $m_{ν_τ} < ~190$ KeV for $T_{\rm QCD}= 200$ MeV. The constraint $Δk_ν~<0.1~$ leads to the much stronger bound $m_ν<10$ KeV for both species of neutrinos.