Chiral dynamics of the two Λ(1405) states

Using a chiral unitary approach for the meson-baryon interactions, we show that two octets of J(pi) = 1/2(-) baryon states, which are degenerate in the limit of exact SU(3) symmetry, and a singlet are generated dynamically. The SU(3) breaking produces the splitting of the two octets, resulting in the case of strangeness S = -1 in two poles of the scattering matrix close to the nominal Lambda(1405) resonance. These poles are combinations of the singlet state and the octets. We show how actual experiments see just one effective resonance shape, but with properties which change from one reaction to another. (C) 2003 Elsevier B.V. All rights reserved.