Angular momentum and light-front scattering theory

The role of the Leutwyler and Stern spin operator in the angular momentum analysis of light-front scattering theory is analyzed. The equations of formal scattering theory are transformed to the picture using the unitary operator C() recently developed by the author. This operator depends on the two angles which determine the direction of the three-vector part of a lightlike four-vector . It is shown that an invariant version of light-front perturbation theory developed earlier by the author is related to the standard theory by the unitary operator C(). It is also shown how to carry out a partial-wave analysis of the Lippmann-Schwinger-like equations obtained by summing a subset of the diagrams of this invariant form of light-front perturbation theory. The analysis presented here makes clear that the picture overcomes many of the difficulties due to the interaction dependence of light-front angular momentum operators, in particular the difficulties arising from the fact that the individual diagrams of light-front pertubation theory are not rotationally invariant.