Hard diffractive electroproduction of vector mesons in QCD

TL;DR

The paper develops a leading-log QCD framework for hard diffractive electroproduction of vector mesons, linking exclusive γ* N → V N amplitudes to the gluon density xG(x,Q^2) and the vector meson light‑cone wave function. It systematically analyzes preasymptotic effects such as quark Fermi motion and heavy‑quark masses, demonstrates how Q^2 rescaling and effective scales modify predictions, and compares with available data on ρ and J/ψ production. The work also addresses flavor symmetry restoration, excited-state production, and establishes kinematic bounds for the applicability of the LO approximation, extending to nuclear targets and highlighting significant gluon shadowing in nuclei at small x with implications for high‑energy heavy‑ion physics. Overall, it provides a comprehensive perturbative description of hard diffractive vector meson processes and their sensitivity to the three‑dimensional color structure of hadrons.

Abstract

Hard diffractive electroproduction of longitudinally polarized vector mesons is calculated within the leading $α_s\ln{Q^2\overΛ_{QCD}^2}$ approximation of QCD using the leading order parton densities within the nucleon. Novel QCD features of the production of excited states and of the restoration of flavor symmetry are analyzed. At the onset of the asymptotic regime, our analysis finds an important role of quark Fermi motion within the diffractively produced vector mesons, and we suggest to use this effect to measure the high momentum tail of the wave function of the vector mesons. We deduce a kinematical boundary for the region of applicability of the decomposition of the hard amplitudes over powers of $Q^2$ and/or a limit on the increase of the cross sections of hard processes at small $x$, and briefly analyze its consequences. We also estimate the nuclear attenuation of the diffractive electroproduction of vector mesons and compare with estimates of the shadowing of the longitudinal structure function.

Figures (16)

• Figure 1: Feynman diagrams relevant for the evaluation of the cross section of diffractive electroproduction of longitudinally polarized vector mesons, i.e., the $\gamma_L^* + T \rightarrow V + T$ process, in leading $\alpha_s\ln{Q^2\over {\Lambda_{QCD}^2}}$ approximation.
• Figure 2: Average transverse size of the $q\bar{q}$ components effective in ${\cal A}_{\gamma^*_LN \rightarrow VN}$ of Eq. (\ref{['brep']}) and $\sigma_L$ of Eq. (\ref{['sigmal']}). In our numerical calculations, we used the GRV94 parametrization GRV of the nucleon's gluon density.
• Figure 3: Color-dipole cross section, $\sigma_{q \bar{q} N}(x,b)$ of Eq. (\ref{['sigmabb']}), as a function of the transverse size of the $q\bar{q}$ pair for various values of $x$ and for the GRV94 parametrization GRV of the nucleon's gluon density.
• Figure 4: Average transverse sizes of the $c\bar{c}$ components effective in ${\cal A}_{\gamma^{(*)}N \rightarrow V_c N}$ for $J/\Psi$ and $\Psi'$ photo- and electroproduction.
• Figure 5: Suppression factor, $T(Q^2)$ of Eq. (\ref{['tfactor']}), in $d\sigma_{\gamma^*_LN \rightarrow VN}/dt|_{t=0}$ due to transverse quark Fermi motion for diffractively produced vector mesons built of light quarks.