Towards an understanding of isospin violation in pion-nucleon scattering

We investigate isospin breaking in low-energy pion-nucleon scattering in the framework of chiral perturbation theory. This work extends the systematic analysis of Fettes et al. [Phys. Lett. B 451, 233 (1999)] to the energy range above threshold. Various relations, which identically vanish in the limit of isospin symmetry, are used to quantify isospin breaking effects. We study the energy dependence of the S- and P-wave projections of these ratios and find dramatic effects in the S waves of those two relations which are given in terms of isoscalar quantities only. This effect drops rather quickly with growing center-of-mass energy.