The Λ( 1405 ) resonance as a genuine three-quark or molecular state
The mechanism for the formation of the Λ( 1405 ) resonance is studied in a chiral quark model that includes quark-meson as well as contact (four point) interactions. The negative-parity S -wave scattering amplitudes for strangeness − 1 and 1 are calculated within a unified coupled-channel framework that includes the K N , ¯ K N , πΣ , ηΛ , K Ξ , πΛ , and ηΣ channels and possible genuine three-quark bare singlet and octet states corresponding to 12 − resonances. It is found that in order to reproduce the scattering amplitudes in the S 01 partial wave it is important to include the pertinent three-quark octet states as well as the singlet state, while the inclusion of the contact term is not mandatory. The Laurent-Pietarinen expansion is used to determine the S -matrix poles. Following their evolution as a function of increasing interaction strength, the mass of the singlet state is strongly reduced due to the attractive self-energy in the πΣ and ¯ K N channels; when it drops below the K N threshold, the state acquires a dominant ¯ K N component which can be identified with a molecular state. The attraction between the kaon and