Chiral dynamics and the S11(1535) nucleon resonance

The SU(3) chiral effective lagrangian at next-to-leading order is applied to the S-wave meson-baryon interaction in the energy range around the ηN threshold. A potential is derived from this lagrangian and used in a coupled channel calculation of the πN, ηN, KΛ, KΣ system in the isospin-12, l = 0 partial wave. Using the same parameters as obtained previously from a fit to the low energy KN data it is found that a quasi-bound KΣ-KΛ-state is formed, with properties remarkably similar to the S11(1535) nucleon resonance. In particular, we find a large partial decay width into ηN consistent with the empirical data.