Current-current model for vector-meson photoproduction and related processes

Abstract

We extend the Pomeron-photon analogy (i.e., the idea that the couplings of Pomerons and photons to hadronic matter are similar) to vector-meson photoproduction and related processes, such as $\ensuremath{\rho}$ electroproduction and Compton scattering. To avoid directly coupling the Pomeron to photons, which is awkward in this model, we follow conventional vector-meson-dominance ideas and approximate the photon's hadronic component by the $\ensuremath{\rho}$ meson. We then make a current-current ansatz for the $\ensuremath{\rho}$ elastic scattering amplitude. Spin is fully taken into account. We find that $s$-channel helicity conservation (SCHC) cannot be realized within this framework if the effective current is conserved; SCHC requires that either this current has a nonconserved component or the forward cross section vanishes. If an effective current with two components is introduced---one a conserved electromagnetic central component, and the other a nonconserved peripheral component---then all the main features of $\ensuremath{\rho}$ photoproduction can be accounted for. The same model gives a good qualitative description of $pp$, $\ensuremath{\pi}p$, and $\mathrm{Kp}$ elastic scattering, and of diffractive dissociation; in other words, it provides a unified description of all diffractive phenomena. Both components of the effective current have a simple explanation in terms of the quark-parton model.