Diffraction at HERA and the Confinement Problem

TL;DR

This work analyzes diffraction at HERA to address the confinement problem by contrasting high-energy gamma* p scattering with traditional hadron-hadron processes. It frames the energy rise of cross sections in terms of perturbative QCD radiation for small transverse sizes and a transition to nonperturbative wee partons as the photon size grows, proposing a radiation cloud as the unifying picture. The color-dipole/GBW saturation framework provides a quantitative bridge across short and long distance regimes, explaining both total and diffractive cross sections and suggesting a high-density parton phase as an intermediate step toward confinement. Collectively, the study advances a radiation-based narrative for linking perturbative QCD to confinement and motivates future explorations of saturation and high-density QCD dynamics at small x.

Abstract

We discuss HERA data on the high energy behavior of the total gamma*p cross section and on diffraction in deep inelastic scattering. We outline their novelty in comparison with diffraction in high energy hadron hadron scattering. As a physical picture, we propose an interpretation in terms of QCD radiation at small and large distances: a careful study of the transition between the two extremes represents a new approach to the QCD confinement problem.

Figures (11)

• Figure 1: (a) Section of the hadron-hadron scattering process in the plane transverse to the direction of flight. The circles denote the hadrons, the shaded area the full interaction region (eq.\ref{['eq:bs']}): whereas the size of the hadrons stays fixed, the extension of scattering profile grows with energy; (b) Section of the $\gamma^*p$ scattering process in the transverse direction. The big circle denotes the proton, the black dot the virtual photon which creates a $q\bar{q}$ pair and then builds up its radiation cloud which is denoted by the small open circles.
• Figure 2: $\gamma^* p$ cross section as a function of $W^2$ at various $Q^2$. The values of $Q^2$ are shown on the left side together with the scale factor applied to the data for a better visibility. The full line shows a QCD-fit MRS, the dashed line shows a fit by a model GBW.
• Figure 3: The exponent $\lambda_{tot}$ in the parameteriztion $\sigma_{tot}^{\gamma^* p} \sim (W^2)^{\lambda_{tot}}$, plotted as a function of $Q^2$.The full line shows a QCD-fit MRS, the dashed line shows a fit by a model GBW.
• Figure 4: Space-Time diagram of the elastic $\gamma^* p$ scattering process in the proton rest frame.
• Figure 5: Cross sections for diffractive $\rho$ and $J/\Psi$ production in the $\gamma^* p$ processes. The data are plotted at a scale $Q^2_{eff}$, shown on the left hand side of the plot and defined in the text. On the right hand side the scale factor is shown which was applied to the data at $Q^2_{eff} = 9.8$ and $14.8$ GeV$^2$ for better visibility.