Exclusive hadron production processes in photon-photon collisions provide
important tests of QCD at the amplitude level, particularly as measures of
hadron distribution amplitudes and skewed parton distributions. The
determination of the shape and normalization of the distribution amplitudes has
become particularly important in view of their importance in the analysis of
exclusive semi-leptonic and two-body hadronic B-decays. Interesting two-photon
physics, including doubly-tagged Ξ³βΞ³β reactions, will be
accessible at low energy, high luminosity e+eβ colliders, including
measurements of channels important in the light-by-light contribution to the
muon g--2 and the study of the transition between threshold production
controlled by low-energy effective chiral theories and the domain where
leading-twist perturbative QCD becomes applicable. The threshold regime of
hadron production in photon-photon and e+eβ annihilation, where hadrons
are formed at small relative velocity, is particularly interesting as a test of
low energy theorems, soliton models, and new types of resonance production.
Such studies will be particularly valuable in double-tagged reactions where
polarization correlations, as well as the photon virtuality dependence, can be
studied.Comment: Paper PEP-N W01 presented at the e+ e- Physics at Intermdiate
Energies Workshop, SLAC, April 30-May 2, 2001. References adde
