The neutral pion photoproduction on the proton near threshold

Abstract

The neutral pion photoproduction on the proton near threshold drew tremendous attention of physicists in the context of a possibility that the low energy theorem (LET) might be broken down. A break of the LET implies a break of partially conserved axial current (PCAC) hypothesis. Pion p-wave contribution should be properly subtracted from the total cross section to extract the electric dipole amplitude near threshold and that the experimental results could well be reproduced consistently with the LET if vector meson contributions are taken into account. The results lead to a conclusion that the rescattering of the pion carrying an imaginary momentum below the π+ threshold can play important roles as a physically meaningful process. The experimental results are consistent with the LET and the PCAC hypothesis remains valid.