Chiral Dynamics in Photopion Physics: Theory, Experiment, and Future Studies at the HIγS Facility

We review photopion experiments on the nucleon in the near-threshold region. We compare the results with the predictions of the low-energy theorems of quantum chromodynamics calculated using chiral perturbation theory (ChPT), which is based on the spontaneous breaking of chiral symmetry as well as explicit breaking due to the finite quark masses. As a result of the vanishing of the threshold amplitudes in the chiral limit, the experiments are difficult to perform because the cross sections are small. Nevertheless, the field is now mature in terms of accuracy and sensitivity. We also discuss the accomplishments and limitations of past measurements, as well as future planned experiments at Mainz and HIγS. The technical developments required for the HIγS facility are emphasized. Finally, we show that future experiments will provide even more accurate tests of ChPT and will be sensitive to isospin-breaking dynamics due to the mass difference between the up and down quarks.