Threshold pion photoproduction on nucleons

Pion photo- and electroproduction of the nucleon at threshold is reviewed in theory and experiment. In this region the leading S-wave amplitudes (E₀₊) are predicted by low-energy theorems (LET) based on current conservation and chiral symmetry. While these predictions are in agreement with the data for charged pion production, LET cannot explain the reaction p( gamma , pi ⁰)p over the energy range of the first 10 MeV above threshold. Present calculations involving final state interactions are difficult to reconcile with the requirements of LET and suffer from a strong model dependence due to off-shell formfactors. With the completion of the new electron accelerators with high duty factor and high intensity, new types of coincidence experiments with polarization degrees of freedom will be performed. The authors review the formalism for such reactions and present the multipole decomposition of the various response functions. The new experimental capabilities allow some of the most challenging problems of intermediate energy physics to be attacked, such as the puzzle of neutral pion production at threshold, the electric quadrupole amplitude in the region of the Delta resonance (bag deformation), the Coulomb monopole amplitude near the Roper resonance ('breathing mode'), the production of eta -mesons via the N*(1535) and the production of two pions and more to test consequences of chiral symmetry and the interaction of pions at low energies.