Diffractive Electroproduction *

In the following, the term ‘diffractive electroproduction’ or ‘diffractive deep inelastic scattering (DIS)’ is used to characterize those processes in small-x DIS where, in spite of the high virtuality of the exchanged photon, the proton target remains intact or almost intact. Thus, the term is used synonymously with the expression ‘rapidity gap events in DIS’. Historically, the term diffraction is derived from optics, where it describes the deflection of a beam of light and its decomposition into components with different frequencies. In high energy physics, it was originally used for small-angle elastic scattering of hadrons (Fig. 1a). If one of the hadrons, say the projectile, is transformed into a set of two or more final state particles, the process is called diffractive dissociation or inelastic diffraction. Good and Walker have pointed out that a particularly intuitive physical picture of such processes emerges if the projectile is described as a superposition A +B of different components which scatter elastically off the target [1] (Fig. 1b). Since the corresponding elastic amplitude is different for each component, the outgoing beam will contain a new superposition αA+βB of these components and therefore, in general, new physical states. These are the dissociation products of the projectile. Even at very high energy, the above processes are generically soft, i.e., the momentum transfer is small and the dissociation products have small p⊥. Therefore, no immediate relation to perturbative QCD is apparent.