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

Abstract

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 photoabsorption 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 $T_{\mu \nu }$ 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 $q^2$ behavior of this fixed pole is most easily detected in wide-angle brems-strahlung experiments, though the same mechanism will also give rise to an $s$-wave enhancement independent of the photon masses in $\mathrm{ee}\to \mathrm{ee}\pi \pi$.