Chiral Unitary Approach to Hadron Spectroscopy

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 lowestorder chiral Lagrangian and the N/D method or equivalently the BetheSalpeter equation to implement unitarity. This chiral approach leads to the dynamical generation of the Λ(1405), Λ(1670) and Σ(1620) states for S = −1, the N∗(1535) for S = 0 and the Ξ(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 Λ(1405) and the Λ(1670) resonances with K̄N and KΞ quasibound states, respectively. Our results are found to be incompatible with the measured properties of the Ξ(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 Ξ(1620) resonance as the S = −2 member of the lowest-lying 1/2− octet. The low-energy KN scattering and transition to coupled channels is one of the cases of successful application of chiral dynamics in the baryon sector. The studies of Refs.[1, 2] showed that one could obtain an excellent description of the low-energy data starting from chiral Lagrangians and using the multichannel Lippman-Schwinger equation to account for multiple scattering and unitarity in coupled channels. By including all open channels above threshold and fitting a few chiral parameters of the second-order Lagrangian one could obtain a good agreement with the data at low energies. This line of work was continued in Ref.[3], where all coupled channels that could be arranged from the octet of pseudoscalar Goldstone bosons and the entire baryon ground state octet were included . In Ref.[3] it was demonstrated that using the Bethe-Salpeter equation (BSE) with coupled channels and using the lowest-order chiral Lagrangians,