Chiral Symmetry and s-wave Low-Lying Meson-Baryon Resonances

Abstract

The $s-$wave meson-baryon scattering is analyzed for the isospin-strangeness $I=1/2, S=0$ and $I=0,S=-1$ sectors, in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. For both sectors, four channels have been considered: $πN$, $ηN$, $K Λ$, $K Σ$ and $πΣ$, $\bar K N$, $ηΛ$, $K Ξ$, respectively. The needed two particle irreducible matrix amplitudes are taken from lowest order Chiral Perturbation Theory in a relativistic formalism. There appear undetermined low energy constants, as a consequence of the renormalization of the amplitudes, which are obtained from fits to the available data: elastic $πN $ phase-shifts, $π^- p \to ηn$ and $π^- p \to K^0 Λ$ cross sections and to $πΣ\toπΣ$ mass-spectrum, the elastic $\bar K N \to \bar K N$ and $ \bar K N\to πΣ$ $t$--matrices and to the $ K^- p \to ηΛ$ cross section data. The position and residues of the complex poles in the second Riemann sheet of the scattering amplitude determine masses, widths and branching ratios of the $S_{11}-$ $N$(1535) and $-N$(1650) and $S_{01}-$ $Λ$(1405) and $-Λ$(1670) resonances, in reasonable agreement with experiment. A good overall description of data, from threshold up to around 2 GeV is achieved despite the fact that three-body channels have not been explicitly included.

5 Pages, 2 figures, invited contribution to Focus Session on Nature of Threshold N*, to be published in Proceedings of Nstar 2002, Pittsburgh, USA, October 9-12, 2002 (World Scientific)