Neutrino conversions in hot plasma

Abstract

We discuss the excitation of sterile neutrinos in the early universe using general quantum kinetic equations, which also incorporate a possible primordial magnetic field $B$. We find a new contribution to the excitation propability, which has its origin in the shrinkage of the spin vector. In the absence of $B$ nucleosynthesis implies the constraint $\mid \Delta m^2\mid {\sin }^{2}2{\theta }_0\text{lsim}1.6\times 10^{-6}{\mathrm{eV}}^2$ which is more restrictive than previous estimates. We also present examples of possible stringent limits for $B\ne 0$.

Comments: 22 pages LaTeX, 1 uuencoded figure in a separate file