High momentum diffractive processes and hadronic structure

High momentum diffractive processes can be used to study the internal structure of hadrons. This structure is described using the light-cone formalism and the concept of light-cone wave functions (LCWFs). These exclusive observables are related to the more inclusive ones, the structure functions and form factors. Experimental methods to measure the LCWFs through measurements of form factors have proven rather insensitive and alternative approaches are necessary. Measurements of LCWFs using diffractive dissociation are differential and determine the momentum distributions of the valence partons in the hadron. They provide wave function information at the amplitude level. The concept of these measurements relies on the factorization of soft and hard processes. Recent measurements of the pion and the photon LCWFs by diffractive dissociation are described. The measurement of the pion LCWF has been extensively and critically discussed by several authors. These discussions are summarized and conclusions are drawn. These experiments allow observation of the transition from perturbative to non-perturbative QCD regimes. This transition is related to the soft components of the LCWF and to chiral symmetry breaking. The phenomenon of color transparency (CT) is an important component of the experimental approach and is closely related to the LCWF description of the hadronic structure. Observation of this effect in diffractive dissociation of pions interacting with nuclear targets is described and compared with other studies of CT. We finally discuss several future directions in this field.