SLAC-PUB-1012 (TH) and (EXP) January 1972 PHENOMENOLOGY OF PHOTON PROCESSES, VECTOR DOMINANCE AND CRUCIAL TESTS FOR PARTON MODELS* Stanley J. Brodsky, Francis E. Closet and J. F. Gunion Stanford Linear Accelerator Center Stanford University, Stanford, California 94305 ABSTRACT We discuss the phenomenological consequences of parton models for photon processes. In particular, the “breakdown” of vector meson domi- nance 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. (Submitted to Phys. Rev. ) * Work supported in part by the U. S. Atomic Energy Commission. t NATO Fellow, 1970-1972.