Cosmological abundances of right-handed Dirac neutrinos

Abstract

The equilibration of the right-helicity states ${\nu }_{+}$ of light Dirac-neutrinos is discussed. I point out that the ${\nu }_{+}$ production rate is enhanced by weak gauge boson pole effects so that the right-helicity component of ${\nu }_{\tau }$ is brought into equilibrium at $T\simeq 10$ GeV independently of the initial abundance, provided $m_{ν_τ}\gsim 10$ keV. Neutrino spin flip in primordial magnetic fields and the resulting bound on ${\mu }_{\nu }$ is also discussed.

Talk given at the Neutrino '96 Conference. 5 pages, LaTeX, 3 ps figures included