2 00 2 On the spin , parity and nature of the Ξ ( 1620 ) resonance

Using a unitary extension of chiral perturbation theory with a lowest-order swave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness S = −2 sector. A scattering-matrix pole is found around 1606 MeV which corresponds to an s-wave Ξ resonance with JP = 1/2−. We identify this resonance with the Ξ(1620) state, quoted in the Particle Data Book with I = 1/2 but with unknown spin and parity. The addition of the S = −2 state to the recently computed Λ(1670), Σ(1620) and N(1535) states completes the octet of JP = 1/2− resonances dynamically generated in this chiral unitary approach. Since the discovery of the first baryon resonance, the ∆(1232), over fifty years ago, there has been a persistent question regarding the nature of such resonances: Are these genuine states that appear as bare fields in the driving term of the meson-baryon scattering matrix? Or can they be generated dynamically, i.e., by iterating an appropriate nonpolar driving term to all orders, just like the deuteron appears as a bound state when the nucleon-nucleon interaction is solved to all orders? Early attempts by Chew and Low to generate the ∆(1232) by iterating a crossed nucleon pole term were eventually overtaken by the success of the SU(3) quark models which established the ∆(1232) as part of the