On the size of isospin violation in low-energy pion-nucleon scattering

We present an analysis of isospin-breaking effects in threshold pion-nucleo n scattering due to the light quark mass difference and the dominant virtua l photon effects. We discuss the deviation from various relations, which ar e exact in the isospin limit. The size of the isospin-violating effects in the relations involving the isovector pi N amplitudes is typically of the o rder of one percent. We also find a new remarkably large effect in an isosc alar triangle relation connecting the charged and neutral pion scattering o ff protons. (C) 1999 Elsevier Science B.V. All rights reserved.