Extracting the dipole cross-section from photo- and electro-production total cross-section data

TL;DR

The paper develops a color-dipole framework to extract the dipole–proton cross-section for fixed transverse size from high-energy γp data, using both electroproduction and photoproduction and constrained by photon-dissociation to total cross-section ratios. It introduces a two-term σ(s,r) with soft and hard components and uses photon light-cone wavefunctions; initial QED wavefunctions overestimate large-dipole contributions, leading to unphysical cross-sections, which motivates a phenomenological confinement-inspired modification to the wavefunction. The fits to HERA and fixed-target data yield a positive, energy-dependent dipole cross-section with a reasonable soft/hard separation and diffractive ratios in line with experiment, and including charm modestly shifts the balance without compromising the overall description. The results challenge the necessity of saturation in the energy variable and provide a framework for predicting diffractive dissociation, vector-meson production, and deeply virtual Compton scattering from the extracted dipole cross-section.

Abstract

We report on a successful attempt to extract the cross-section for the high-energy scattering of colour dipoles of fixed transverse size off protons using electroproduction and photoproduction total cross-section data, subject to the constraint provided by the ratio of the overall photon dissociation cross-section to the total cross-section.

Figures (11)

• Figure 1: The diffractive process from a mixed position-momentum viewpoint. Transverse components are spatial; non-transverse components are light cone momenta.
• Figure 2: The weight function $f(r) G(r) / r$ for different $Q^{2}$ (fit I). The peak at low $Q^{2}$ represents the modification to the photon wave function.
• Figure 3: Representative sample of fitted data points for the total cross-section $\sigma_{\gamma p}^{tot}$ compared with curves calculated from the parameterised dipole cross-section for different $Q^{2}$ values (fit I).
• Figure 4: Ratio of the overall singly dissociative diffractive cross-section to the total cross-section for fit I (solid line) and fit II (dotted line).
• Figure 5: The dipole cross-section at different energies (fit I).