Neutral pion photoproduction on the proton near threshold and the chiral symmetry

The recent precise measurements of the neutral pion photoproduction on the proton near threshold seems to be inconsistent with the predictions of the low-energy theorems on photoproduction of pions. In this note, we have sought to resolve this problem, in the framework of an approximate SU(2)×{}SU(2) symmetry of the pion-nucleon dynamics, by incorporating the contribution from the nucleon matrix elements of the equal-time commutator between the divergence of the axial-vector current and the electromagnetic current, which was introduced earlier by Furlan et al. We find that a mean value of the nonstrange quark mass m^=1/2(mᵤ+md)=(7±{}2) MeV, leads to an estimate of the symmetry breaking terms compatible with experiment.