We give an overview of the physics motivation and evolution of the neutral
pion photoproduction measurements in the threshold region conducted in the A2
collaboration at MAMI. The latest two experiments have been performed with the
almost 4-pi Crystal Ball detector. The first was with a linearly polarized
photon beam and unpolarized liquid-hydrogen target. The data analysis is now
complete and the linearly polarized beam asymmetry along with differential
cross sections provide the most stringent test to date of the predictions of
Chiral Perturbation Theory and its energy region of convergence. More recently
a measurement was performed using both circularly polarized photons and a
transversely polarized butanol frozen-spin target, with the goal of extracting
both the target and beam-target asymmetries. From these we intend to extract
pi-N scattering sensitive information for the first time in photo-pion
reactions. This will be used to test isospin conservation and further test
dynamics of chiral symmetry breaking in QCD as calculated at low energies by
Chiral Perturbation Theory.Comment: 8 pages, 5 figures, submitted to EPJ Special Topics as part of the
proceedings of the Conclusive Symposium of Collaborative Research Centre 44
