Experimental tests of chiral perturbation theory

In this paper the current status of the threshold photo- and electroproduction of neutral pions as test of Chiral Perturbation Theory is summarized. The combination of differential cross-section data with polarized photon asymmetries allows to determine a complete set of s- and p-wave amplitudes at threshold for the photoproduction case. These extracted amplitudes are in good agreement with predictions and fits of Heavy Baryon Chiral Perturbation Theory. On the other hand, new data on pion electroproduction shows a significant deviation from the predicted cross-sections at a four-momentum transfer of Q2=0.05 GeV2/c2. For the coherent pion production from the deuteron the photoproduction data agree with Chiral Perturbation Theory. The former reported severe disagreement between the electroproduction s-wave amplitude at a four-momentum transfer of Q2=0.1 GeV2/c2 and first calculations seems to be resolved now. (orig.)