Evidence for Two Λ(1405) Resonance States

The K − p → π 0 π 0 Σ 0 reaction is studied within a chiral unitary model. The distribution of π 0 Σ 0 states forming the Λ(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of Γ = 38 MeV. We use these data in combination with those of the π − p → K 0 πΣ reaction and elements of chiral unitary theory to prove that there are two Λ(1405) states instead of one as so far assumed. The history of the Λ(1405) as a dynamical resonance generated from the interaction of meson baryon components in coupled channels has experienced a boost within the context of unitary extensions of chiral perturbation theory (U χP T) [1, 2, 3, 4, 5], where the lowest order chiral Lagrangian and unitarity in coupled channels generates the Λ(1405) and leads to good agreement with the K − p reactions. The surprise, however, came with the realization that there are two poles in the neighborhood of the Λ(1405) both contributing to the final experimental invariant mass distribution [3, 4, 5, 6, 7, 8, 9]. The properties of these two states are quite different, one has a mass around 1390 MeV, a large width of about 130 MeV