Diffraction and the Pomeron

In hadron-hadron scattering, interactions are classified by the characteristics of the final states. In elastic scattering, both hadrons emerge unscathed and no other particles are produced. In diffractive scattering, the energy transfer between the two interacting hadrons remains small, but one (single dissociation) or both (double dissociation) hadrons dissociate into multi-particle final states, preserving the quantum numbers of the associated initial hadron. The remaining configurations correspond to inelastic interactions. The first interpretation of diffraction, due to Good and Walker [1], was that different components of the projectile were differently absorbed by the target, leading to the creation of new physical states. This was the first indication for the composite nature of hadrons. In the Regge theory of strong interactions [2], diffraction is the result of exchanging a universal trajectory with the quantum numbers of the vacuum, the (soft) Pomeron, IP , introduced by Gribov [3]. In the language of Quantum Chromodynamics, the candidate for vacuum exchange with properties similar to the soft Pomeron is two gluon exchange [4, 5]. As a result of interactions between the two gluons, a ladder structure develops. In perturbative QCD, the properties of this ladder depend on the energy and scales involved in the interaction, implying its non-universal character. In the high-energy limit, the properties of the ladder have been derived for multi-Regge kinematics and the resulting exchange is called the (hard) BFKL pomeron [6, 7, 8]. A renewed interest in diffractive scattering followed the observation of a copious production of diffractive-like events in deep inelastic scattering (DIS) at the HERA ep collider [9, 10] as well as the earlier observation of jet production associated with a leading proton in pp at CERN [11]. The presence of a large scale opens the possibility of studying the partonic structure of the diffractive exchange as suggested by Ingelman and Schlein [12] and testing QCD dynamics. Moreover, the study of diffractive scattering offers a unique opportunity to understand the relation between