Neutral pion photoproduction off nucleons revisited

TL;DR

This paper revisits threshold neutral pion photoproduction using heavy baryon chiral perturbation theory, highlighting slow convergence of the electric dipole amplitude and proposing new low-energy theorems for P-waves.

Contribution

It introduces new, rapidly converging low-energy theorems for P-waves and discusses polarization observables and neutron reaction predictions.

Findings

Electric dipole amplitude $E_{0+}$ shows slow convergence.

New low-energy theorems for P-waves are proposed.

Predictions for $ ext{γ}n o ext{π}^0 n$ are provided.

Abstract

We investigate threshold pion photoproduction in the framework of heavy baryon chiral perturbation theory. We give the expansion of the electric dipole amplitude $E_{0+}$ to three orders in $\mu$, the ratio of the pion to nucleon mass, and show that it is slowly converging. We argue that this observable is not a good testing ground for the chiral dynamics of QCD. In contrast, we exhibit new and fastly converging low--energy theorems in the P--waves which should be used to constrain the data analysis. We also discuss the importance of polarization observables to accurately pin down certain multipoles and give predictions for the reaction $\gamma n \to \pi^0 n$.