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Exclusive diffractive processes and the quark substructure of mesons

Exclusive diffractive processes on the nucleon are investigated within a model in which the quark-nucleon interaction is mediated by Pomeron exchange and the quark substructure of mesons is described within a framework based on the Dyson-Schwinger equations of QCD. The model quark-nucleon interaction has four parameters which are completely determined by high-energy $πN$ and $K N$ elastic scattering data. The model is then used to predict vector-meson electroproduction observables. The obtained $ρ$- and $ϕ$-meson electroproduction cross sections are in excellent agreement with experimental data. The predicted $q^2$ dependence of $J/ψ$-meson electroproduction also agrees with experimental data. It is shown that confined-quark dynamics play a central role in determining the behavior of the diffractive, vector-meson electroproduction cross section. In particular, the onset of the asymptotic $1/q^4$ behavior of the cross section is determined by a momentum scale that is set by the current-quark masses of the quark and antiquark inside the vector meson. This is the origin of the striking differences between the $q^2$ dependence of $ρ$-, $ϕ$- and $J/ψ$-meson electroproduction cross sections observed in recent experiments.