4 Ju n 20 02 INCLUSIVE DIFFRACTION AT HERA

The diffractive dissociation of virtual photons, γ * p → Xp, has been studied in ep interactions at HERA with the H1 and ZEUS detectors. The data are presented in terms of the diffractive structure function F D 2 and of the diffractive cross sections dσ/dt and dσ/dMX. The t-dependence of the cross section is measured. The Pomeron intercepts extracted from diffractive and inclusive ep interactions are compared. The result is further interpreted studying the dependence of the ratio between the diffractive to the inclusive cross section on the photon-proton center of mass energy. Recent data on the Q 2 dependence of the diffractive cross section at 0.17 < Q 2 < 0.70 GeV 2 constrain the transition in diffraction from the perturbative high Q 2 region to the photoproduction limit. The diffractive dissociation of real photons, γp → Xp, can be treated in complete analogy with hadron-hadron diffractive interactions and therefore can be described by Regge phenomenology. Whithin this framework, diffractive reactions at high energy are dominated by the exchange of a trajectory with the quantum numbers of the vacuum, referred to as the " Pomeron " trajectory. By contrast HERA allows the investigation of the partonic nature of diffraction in deep-inelastic scattering (DIS) using virtual photons as probes. The most general HERA diffractive DIS process is shown in Fig. 1. Figure 1: The generic HERA diffractive process of the type γ * p → XY. A photon of virtuality Q 2 (coupled to the electron) interacts with a proton at a center of mass energy W and squared four momentum transfer t to produce two distinct hadronic systems of masses M X and M Y , respectively. The system X is usually reconstructed in the central detector and is well separated in rapidity from the proton remnant system Y which, for small masses M Y , escapes through the forward beam hole of the detector. The photon dissociative processes, where the proton remains intact (Y = p), are selected at HERA either by requiring an absence of particles in the forward region of the central detector (" rapidity