Exclusive Diffractive Final States in Electron-proton Collisions

The diffractive scattering is a process where the colliding particles scatter at very small angles and without any color flux in the final state. This involves a propagator carrying the vacuum quantum numbers, called Pomeron and described, in the soft regime, within the Regge theory. The discovery of a big amount of diffractive events in DIS regime provided a hard scale which can be varied over a wide range and therefore it is an ideal testing for QCD models. In particular, the diffractive production of Vector Mesons (VMs) and real photons in ep collisions allows to study the transition from the soft to the hard regime in strong interactions. The hard regime (high energy and low Bjorken-x) is dominated by the exchange of a hard Pomeron well described by perturbative QCD (pQCD), while at low-x the interaction is well described within the Regge phenomenology. Indicating with Q the virtuality of the exchanged photon and with M the square mass of the produced VM, HERA data suggested a universal hard scale, Q +M, for the diffractive exclusive pruduction of VM and real photons, which indicates the transition from the soft to the hard regime.