Fine and hyperfine interaction on the light cone

Abstract

The formalism for the spin interactions in the front form (light-cone) is re-phrased in terms of an instant form formalism. It is shown how to unitarily transform the Brodsky-Lepage spinors to Bj{\o}rken-Drell spinors and to re-phrase the so called spinor matrix in terms of the interactions one is familiar with from atomic and Dirac theory. -- One retrieves the (relativistic) kinetic correction, the hyperfine and the Darwin term which acts even when wave function is spherically symmetric. One also retrieves angular momentum dependent terms like the spin-orbit interaction in a relativistically correct way; and one obtains additional terms which thus far have not been reported particularly various $\vec L^2$-dependent terms. Since the approach includes the full retardation, one gets additional, thus far unknown terms. The differ from atomic and Dirac theory, since there only that part of the vector potential is usually included which is generated by the atomic nucleus. Quite on purpose, the paper is kept formal.