Light-cone representation of the spin and orbital angular momentum of relativistic composite systems

Abstract

The matrix elements of local operators such as the electromagnetic current, the energy momentum tensor, angular momentum, and the moments of structure functions have exact representations in terms of light-cone Fock state wavefunctions of bound states such as hadrons. We present explicit light-cone wavefunctions for the two-particle Fock state of the fermion in QED and the Yukawa theory which preserve all the Lorentz properties and give explicit realization of the spin sum rules. The role of orbital angular momentum in understanding the “spin crisis” problem for relativistic systems is clarified. We show how the perturbative structure can be used to simulate much more general bound state systems while preserving all Lorentz properties. We thus obtain a theoretical laboratory to test the consistency of formulae which have been proposed to probe the spin structure of hadrons.