The effect of a magnetic field on $n \to p+e^-+\bar{\nu_e}$ and $e^-+p \to n + \nu_e$

It is shown that in the decay of a set of neutrons, as a whole unpolarized in a magnetic field, produces a negative mean value for the projection of the antineutrino momentum on the field direction as calculated per nucleon if the magnetic field is weak or if the initial neutron distribution over the spin states is restored more rapidly than the decay occurs, while it becomes positive in a strong field if the recovery of the initial spin-state distribution is much slower than the decay. An analogous situation occurs in the reaction $e^-+p \to n+\nu_e$.