Deep inelastic diffractive scattering at HERA

It is well known that soft hadron-hadron scattering can be described by Regge phenomenology, which models the total, elastic and single and double diffractive dissociation cross sections in terms of reggeon and pomeron exchange [1]. At high energies, it is the pomeron exchange mechanism that dominates the total cross section [2]. However, until recently, our understanding of the pomeron in terms of Quantum Chromodynamics (QCD) [3] remained fragmentary, at best. The observation of events with a large rapidity gap in the hadronic final state at HERA [4], which are attributed to diffractive dissociation of (virtual) photons, led to a renewed interest in the study of the underlying dynamics of diffraction. In deep inelastic ep scattering, the long hadronic lifetime of the photon at small Bjorken-x allows to make the link with diffractive dissociation in hadron-hadron scattering, while the presence of one or more hard scales, such as the (variable) virtuality Q, enables perturbative calculations in QCD. In the framework of QCD, it is tempting to attribute a partonic structure to the pomeron [5]; an approach which proves to be very successful in describing various aspects of diffractive virtual photon disscociation. QCD calculations indicate, however, that the pomeron does not exhibit a universal behaviour over the full kinematic range. Indeed, experimentally a transition is observed when going from the soft to the hard scattering