We examine the meson-baryon interaction in the strangeness S = −1 sector using an effective chiral Lagrangian. Potentials are derived from this Lagrangian and used in a coupled-channel calculation of the low energy observables. The potentials are constructed such that in the Born approximation the s-wave scattering amplitude is the same as that given by the effective chiral Lagrangian, up to order q 2 . Comparison is made with the available low energy hadronic data of the coupled K − p, Σπ, Λπ system, which includes the Λ(1405) resonance, K − p elastic and inelastic scattering, and the threshold branching ratios of the K − p decay. Good fits to the experimental data and estimates of previously unknown Lagrangian parameters are obtained. *

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. Potentials are derived from this lagrangian and used in a coupled channel calculation of the πN , ηN , KΛ, KΣ system in the isospin-1/2, l = 0 partial wave. Using the same parameters as obtained from a fit to the low energy KN data it is found that a quasi-bound KΣ-state is formed, with properties remarkably similar to the S 11 (1535) nucleon resonance. In particular, we find a large partial decay width into ηN consistent with the empirical data.

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We investigate the utility of the K+ meson as a probe of effective-size modifications (" swelling") of nucleons in the nuclear medium. The K-nucleus elastic and total cross sections are found to be sensitive to the density-dependent effective masses of the vector mesons (p, tv) whose exchange is responsible for the dominant part of the low-energy K-nucleon interaction. This density dependence is estimated in several ways and used to construct a K+-nucleus optical potential. Such a model is consistent with the K++ '2C data at 800 MeV/c, and provides a physical picture of the "swelling" mechanism in terms of the medium modifications of the meson cloud of the nucleon.