Spin structure of compton scattering at high energies

Abstract

In preceding papers (L2) a model for the diffractive photon-induced processes a t high energies was developed and an analysis of the most important reactions was carried out. The description was based on a multiehannel impact-parameter formalism which provided a unified picture of the elastic, i.e. Compton process, as well as the inelastic diffraction dissociation channel (like vector-meson photoproduction) at high energies. The analysis of Compton scattering was carried throughout neglecting spin because of lack of appropriate experimental information concerning the helieity structure of the reaction (3). However, since in the case of vector-meson photoproduction there is an important amount of knowledge on the density matrix elements of the decaying vector meson (especially for the p0) and, furthermore, one possesses a lot of information from experiments with polarized photon beams, the model was enlarged in order to incorporate these facts into the actual analysis of data. A specific feature of the model is the inherent possibility of existence in the asymptotic amplitude of a helicity-fiip component which is absent in the usual descriptions of diffraction. Moreover, this Pomeranchuk flip amplitude was supposed to completely dominate the flip components coming from secondary Regge exchanges and, indeed, this assumption was subsequently tested in the analysis of ~0 photoproduction, where, as is well known, the Pomeranchuk contr ibution is the only exchange present in the production mechanism (4). In fact, a better fit to the data required a substantial amount of helicityflip present at the nucleonic vertex. Also in po photoproduction the best agreement with the differential-cross-section data was achieved imposing a flipping pomeron at the nucleon vertex, whose coupling strength turned out to be of the same order as in ~o photoproduetion. The lack of experiments with polarized targets does not allow us to directly check the actual existence of such a term since this amplitude cannot be separated with present experiments from the nonflip one. Owing to the fact that great progress has been recently made in obtaining highly polarized ((70--80)%) targets,