Covariant equations of motion, for a charged particle with a magnetic dipole moment

Abstract

The Hamiltonian for a Dirac particle with anomalous magnetic moment in an electromagnetic field is transformed to even form up to terms linear in the coupling constant and without derivatives of the field. The even parts of the position and spin operators are derived by imposing conditions of covariance. Covariant equations of motion and of spin are then deduced; they turn out to have the same form as the classical equations for a composite particle with magnetic dipole moment. (The magnetodynamic effect for a particle in a time-dependent field is shown to contain the vector product of the electric field and the anomalous magnetic moment only.)