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

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. The authors illustrate all of these properties by giving explicit light-cone wavefunctions for the two-particle Fock state of the electron in QED, thus connecting the Schwinger anomalous magnetic moment to the spin and orbital momentum carried by its Fock state constituents. The authors also compute the QED one-loop radiative corrections for the form factors for the graviton coupling to the electron and photon. The authors then generalize these results to arbitrary composite systems, giving explicit realization of the spin sum rules and other local matrix elements. The role of orbital angular momentum in understanding the ''spin crisis'' problem for relativistic systems is clarified. The authors also prove that the anomalous gravitomagnetic moment B(0) vanishes for any composite system. This property is shown to follow directly from the Lorentz boost properties of the light-cone Fock representation and holds separately for each Fock state component. The authors show how the QED perturbative structure can be used to model bound state systems while preserving all Lorentz properties. The authors thus obtain a theoretical laboratory to test the consistency of formulae which have been proposed to probe the spin structure of hadrons