Diffractive electroproduction of light vector particles: leading Fock-state contribution in the presence of significant higher Fock-state effects

Abstract

We study exclusive diffractive production of vector mesons and photon using the color dipole model with leading Fock state light front wave functions derived from Dyson Schwinger and Bethe Salpeter equations. New results for the $\phi$ meson and real photon are presented. Without data fitting, our calculation well matches HERA data in certain kinematical domains. The key finding of this paper is that in a color dipole model study for $\rho/\gamma$ and $\phi$, where light quarks are involved, the leading $q\bar{q}$ approximation is valid only when $Q^2$ exceeds $20$ and 10 GeV$^2$ respectively, unlike $J/\psi$ which can be well described for $Q^2\approx 0$ GeV$^2$. This underscores the special role of $\phi$ electroproduction in color dipole picture: it strikes a balance between the large dipole size typical of light mesons and the smaller size associated with high $Q^2$ photons, making it potentially well suited for probing gluon saturation effects.