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

Using a unitary extension of chiral perturbation theory with a lowest-order s-wave 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 $\Xi$ resonance with $J^P=1/2^-$. We identify this resonance with the $\Xi(1620)$ 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 $\Lambda(1670)$, $\Sigma(1620)$ and N(1535) states completes the octet of $J^P=1/2^-$ resonances dynamically generated in this chiral unitary approach.