Hard exclusive reactions and hadron structure

The generalized Bjorken regime of exclusive reactions opens new ways to explore the hadron structure. We shortly review the present status of this domain where generalized parton distributions, generalized distribution amplitudes and transition distribution amplitudes describe various aspects of confinement physics.Comment: 7 pages LATEX, 5 figures, invited talk at the Vth Conference on Hadronic Physics held at ICTP, Trieste, Italy, May 200

Exclusive production of pentaquarks in the scaling regime

We investigate two exclusive reactions with a Theta+ pentaquark in the final state: electroproduction of a K meson on the nucleon, and K+ scattering on a neutron target producing a lepton pair. These reactions offer unique opportunities to investigate the structure of pentaquark baryons at parton level. We discuss the generalized parton distributions for the N --> Theta+ transition and give the leading order amplitude for these processes in the Bjorken regime.Comment: 5 pages in LATEX, 1 .eps figure, talk at the Baryons04 Conference, October 25-29, 2004, Palaiseau, Franc

Exclusive electroproduction and off-diagonal parton distributions

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.Comment: 7 pages, 3 figures, LaTeX2e. Talk given at the Fourth Workshop on Quantum Chromodynamics, Paris, France, 1-6 June 199

Exclusive hadronization in gamma* gamma -> pi pi

We perform a QCD analysis of exclusive two-meson production in gamma* gamma collisions in the kinematical domain of large photon virtuality Q^2 and small hadronic invariant mass W^2. In these kinematics the amplitude factorizes into a perturbative subprocess gamma* gamma -> q qbar and a two-meson distribution amplitude. This opens the way to study new hadronic matrix elements related with the physics of hadronization and confinement.Comment: 5 pages, 1 figure. Talk given at PHOTON 99, Freiburg, Germany, 23-27 May 1999. Eq. (8) correcte

Polarisation in deeply virtual meson production

We discuss two aspects of polarisation in hard exclusive meson production: the leading-twist selection rule for the meson helicity, and the different partial waves of a (pi pi)-pair which may or may not be due to the decay of a rho.Comment: 9 pages, 1 figure. Talk given at the Workshop on Exclusive and Semiexclusive Processes at High Momentum Transfer, Jefferson Lab, May 1999. Remark added on the transition from two gluons to a pion pai