New sum rule for photoproduction amplitudes based on local current commutators

A sum rule for isovector Compton scattering, previously derived using dispersive techniques, is rederived using the + + light-cone commutator. The (nontrivial) limit of the sum rule at ${q}^{2}=0$ is given, and is found to be in excellent agreement with experiment. The deep-inelastic limit of the sum rule is discussed in the context of the covariant parton model, where it is shown that the free-field realization is untenable.