Magnetic Moment of a Massive Neutrino and Neutrino-Spin Rotation

Abstract
A massive Dirac neutrino has a magnetic moment, which causes its spin to precess in a magnetic field. This reduces the effective weak cross sections for relativistic neutrinos. An estimate on the basis of phenomenological considerations as well as the standard electroweak theory indicates that massive neutrinos from supernovae and neutron stars may contain significant mixtures of negative- and positive-helicity states.