Chiral unitary approach to hadron spectroscopy

Abstract

The s-wave meson-baryon interaction in the $S=-1$, $S=0$ and $S=-2$ sectors is studied by means of coupled channels, using the lowest-order chiral Lagrangian and the N/D method or equivalently the Bethe-Salpeter equation to implement unitarity. This chiral approach leads to the dynamical generation of the $\hat{I}›(1405)$, $\hat{I}›(1670)$ and $\hat{I}\pounds (1620)$ states for $S=-1$, the $N^{\ast }(1535)$ for $S=0$ and the $\hat{I}ž(1620)$ for $S=-2$. We look for poles in the complex plane and extract the couplings of the resonances to the different final states. This allows identifying the $\hat{I}›(1405)$ and the $\hat{I}›(1670)$ resonances with $\bar{K}N$ and $K\hat{I}ž$ quasibound states, respectively. Our results are found to be incompatible with the measured properties of the $\hat{I}ž(1690)$ resonance, thus ruling this state out as the remaining member of this octet of dynamically generated resonances. We therefore assign $1/2^{-}$ for the spin and parity of the $\hat{I}ž(1620)$ resonance as the $S=-2$ member of the lowest-lying $1/2^{-}$ octet.

10 pgs. 4 figs. Talk at the Workshop on partial wave analysis of meson and baryon systems, Pittsburgh, June 2002