Electromagnetic Deflection of Spinning Particles

We show that it is possible to obtain self-consistent and physically acceptable relativistic classical equations of motion for a point-like spin-half particle possessing an electric charge and a magnetic dipole moment, directly from a manifestly covariant Lagrangian, if the classical degrees of freedom are appropriately chosen. It is shown that the equations obtained encompass the well-tested Lorentz force and Thomas–Bargmann–Michel–Telegdi spin equations, as well as providing a definite specification of the classical magnetic_dipole force_, whose exact form has been the subject of recent debate. Radiation reaction—the force and torque on an accelerated particle due to its self-interaction—is neglected at this stage.