S=-1 meson-baryon unitarized coupled channel chiral perturbation theory and the S01 resonances Λ(1405) and -Λ(1670) -: art. no. 076009

The s-wave meson-baryon scattering is analyzed for the strangeness S=-1 and isospin I=0 sector in a Bethe-Salpeter coupled channel formalism incorporating chiral symmetry. Four channels have been considered: piSigma, (K) over barN, etaLambda, and KXi. The required input to solve the Bethe-Salpeter equation is 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 piSigma-->piSigma mass spectrum, to the elastic (K) over barN-->(K) over barN and (K) over barN-->piSigma t matrices and to the K(-)p-->etaLambda cross section data. The position and residues of the complex poles in the second Riemann sheet of the scattering amplitude determine the masses, widths, and branching ratios of the S-01 resonances Lambda(1405) and -Lambda(1670), in reasonable agreement with experiment. A good overall description of the data, from the piSigma threshold up to 1.75 GeV, is achieved despite the fact that three-body channels have not been explicitly included.