On polarization effects in high-energy vector-meson photoproduction

Abstract

In a recent paper (1) an analysis of vcetor-meson photoproduetion at high energies was presented in terms of a multiehannel impact-parameter formalism which provided a unified description of all diffractive photon-induced reactions. I t was argued there that a helieity-flip component of the asymptotic part of the amplitude, i.e. the pomeron, may well be present in the proposed unitarized multiple-scattering picture of diffraction and it was indeed subsequently found in the analysis of 7 ~ and p~ data. As is well known, ~0 photoproduction is an ideal testing ground for the properties of the Pomeranchuk contribution since no other Regge exchanges are supposed to contr ibute there (2). I t was shown that for the c~~ amplitude (and for the i~ ~ amplitude) there is a substantial amount of helicity flip at the proton vertex (*). Of course, this model-dependent result has to be confirmed by direct measurement of the relevant density matrix elements. Unfortunately, the present data cannot provide information about this point because of lack of appropriate data on polarization experiments. One is therefore forced to use the available differential cross-section data in order to get information on this particular helicity amplitude with flip at the nucleon-~nucleon transition. I t seems therefore worth-while to discuss the possible experiments with polarized beams and/or polarized target which could directly test our rather indirect finding of a violation of SCIIC (s-channel helicity conservation) (a) for the recoiling proton in ~o (po) photoproduction. In view of the recent progress in preparing polarized targets possessing polarizations up to (70--80)~o it is feasible to carry out such experiments in the near future. Let us represent the state of polarization of the incoming nucleon and of the photon by the spin density matrices (4.5)