Pion photoproduction at threshold

The predictions of low energy theorems on pion photoproduction are confronted with the exciting new experimental data near threshold. Isobars in the s- and u-channels and heavier mesons exchanged in the t-channel give only small corrections. The role of rescattering is discussed in some detail. A calculation of the threshold amplitude based on current algebra is able to describe the data but has serious shortcomings concerning gauge and Lorentz invariance. Following a derivation of the low energy theorems from field theory, we find that the discrepancy between experiment and theory could be due to a possible induced pseudoscalar of non-pionic origin in the axial vector current