Diffractive Vector Meson Production at HERA: from Soft to Hard QCD

TL;DR

This review synthesizes HERA measurements of exclusive diffractive vector meson production and frames them within microscopic QCD approaches, notably the color dipole model and $k_t$-factorization, to connect soft and hard diffraction. It emphasizes the scanning radius as the key hard scale, the universal mapping to the gluon density $G(x_g,ar{Q}^2)$, and the interplay between longitudinal and transverse production channels, including SCHNC phenomena. The work highlights evidence for a hard QCD Pomeron with a nontrivial $t$-dependence, the role of skewness and off-forward gluon densities, and the sensitivity to vector meson wavefunctions, especially in radial excitations like $ ho(2S)$ and $ ext{J}/ ho$-type systems. The findings demonstrate how VM production at HERA probes the proton’s gluon structure across a wide range of scales, supporting a two-pole (soft+hard) description of the Pomeron and setting the stage for future NLO $k_t$-factorization and unitarity/saturation studies with implications for hadron structure and diffraction in high-energy processes.

Abstract

Experimental results from HERA on diffractive vector meson production and their theoretical interpretation within microscopic QCD are reviewed with an emphasis on the BFKL color dipole and kt-factorization approaches.

Figures (78)

• Figure 1: The kinematics of DIS
• Figure 2: The unitarity relation between DIS and forward Compton scattering
• Figure 3: Schematic diagram of exclusive vector-meson production in $ep$ interaction, $ep \rightarrow eVp$.
• Figure 4: The unified picture of Compton scattering, diffraction excitation of the photon into hadronic continuum states and into the diffractive vector meson
• Figure 5: The VDM amplitude for the vector meson photoproduction