Phenomenology of Photon Processes, Vector Dominance, and Crucial Tests for Parton Models

We discuss the phenomenological consequences of parton models for photon processes. In particular, the " breakdown " of vector meson dominance in Compton scattering is correlated with its failure in nuclear photo-absorption by showing that the parton model gives rise to a nonshadowed pointlike contribution which occurs only in two-photon processes. Included in this contribution is a piece which corresponds in the general Compton amplitude T PV to a term which is independent of energy and photon masses at fixed t. It is emphasized that failure to observe a contribution with such behavior would have profound consequences for conventional parton models. We predict that this contribution will have only a weak t dependence and will lead to a dominantly real spin conserving amplitude at large t values for Compton scattering. The q2 behavior of this fixed pole is most easily detected in wide angle bremsstrahlung experiments, though the same mechanism will also give rise to an s-wave enhancement independent of the photon masses in ee ee 7r7r.