Neutrino conversions in hot plasma Original Research Article

We discuss the production 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 production probability, which has its origin in the shrinkage of the spin vector. In the absence of B, nucleosynthesis implies the constraint |Δm2| sin2 2θ0 $̌1.6 × 10−6 eV2. We also present examples of possible stringent limits for B ≠ 0.