ISOSPIN VIOLATION IN PION-NUCLEON SCATTERING

We construct the complete effective chiral pion-nucleon Lagrangian in the presence of virtual photons to one loop. As an application, we con sider strong and electromagnetic isospin violation for scattering of n eutral pions off nucleons. We show that for the scattering lengths the se isospin violating terms are of the same size as the purely hadronic ones. We also analyze isospin-violating effects for the sigma-term. T hese can be as large as 10% for the absolute value but are negligible for the shift to the Cheng-Dashen point. (C) 1998 Elsevier Science B.V .