Operator approach to isospin violation in pion photoproduction

Unambiguous isospin violation in the strong interaction sector is a key issue in low energy hadronic physics, both experimentally and theoretically. Bernstein has employed the Fermi-Watson theorem to demonstrate that pion photoproduction is a process where isospin violation in the πN system can be revealed, an approach we review here. Here we propose a general operator approach to the phenomenon in pion photoproduction, thereby providing an analogue for the framework that was proposed for πN scattering by Kaufmann and Gibbs. The resulting set of amplitudes could form the basis for determining the multipole amplitudes for photoproduction. Thus, the so resulting phase shift determination from photoproduction can then be used via the Fermi-Watson theorem to resolve discrepancies in πN phase shift analyses. We point out that casting effective Lagrangian results in terms of our framework would be beneficial. The upcoming polarization experiments are an ideal setting to test our approach, and also to constrain better the isotensor currents which strictly are not forbidden.