Exclusive diffractive processes at HERA within the dipole picture

TL;DR

This work demonstrates that exclusive diffractive processes at HERA can be described within an impact parameter dependent saturated dipole framework, using a single Gaussian proton shape and a DGLAP-evolved gluon density constrained by total DIS data. The b-Sat model, with its minimal parameter set, successfully describes γ*p total and exclusive cross sections, vector meson t-distributions, and DVCS across Q^2, W, and t, indicating a universal transverse gluon density. A complementary b-CGC approach highlights saturation dynamics and non-forward evolution, though it provides a less complete fit to inclusive data. Overall, the results support a universal gluon distribution and a compact proton core, with implications for interpreting transverse structure at high energies and guiding future measurements (e.g., at JLab).

Abstract

We present a simultaneous analysis, within an impact parameter dependent saturated dipole model, of exclusive diffractive vector meson (J/psi, phi and rho) production, deeply virtual Compton scattering and the total gamma* p cross section data measured at HERA. Various cross sections measured as a function of the kinematic variables Q^2, W and t are well described, with little sensitivity to the details of the vector meson wave functions. We determine the properties of the gluon density in the proton in both longitudinal and transverse dimensions, including the impact parameter dependent saturation scale. The overall success of the description indicates universality of the emerging gluon distribution and proton shape.

Figures (28)

• Figure 1: The elastic scattering of a virtual photon on a proton in the dipole representation.
• Figure 2: The elastic scattering amplitude for inclusive DIS (left) and vector meson production (right). For DVCS, the outgoing vector meson in the right-hand diagram is replaced by a real photon.
• Figure 3: Overlap functions \ref{['eq:overt']} and \ref{['eq:overl']} between the photon and vector meson wave functions integrated over $z$ for the three different vector mesons at $Q^2$ values representative of the data.
• Figure 4: Dipole cross section at various impact parameters, as determined in the b-Sat model.
• Figure 5: Top: The total DIS cross section $\sigma_{\rm tot}^{\gamma^*p}$ vs. $W^2$ for different $Q^2$. The data points plotted are from ZEUS Breitweg:2000ynChekanov:2001qu. Bottom: The $\lambda_{\rm tot}$ parameter for inclusive DIS defined by $\sigma^{\gamma^*p}_{\rm tot}\propto (1/x)^{\lambda_{\rm tot}}$. The data points plotted are from ZEUS Breitweg:2000ynChekanov:2001qu and H1 Adloff:2000qk.