Exclusive electroproduction and off - diagonal parton distributions 1

Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale. 1 From diagonal to off-diagonal parton distributions Off-diagonal (also called off-forward or nonforward) parton distributions have enjoyed considerable interest since it was realised that they allow the description of certain exclusive reactions in the framework of QCD factorisation.[1, 2, 3] A good physics example to see how they come about and how they are connected with the usual, diagonal parton distributions , is the virtual Compton scattering amplitude. Let us therefore start with the 1 Talk given by M.