A study of π-nucleon S-wave resonances and the η-nucleon interaction

Abstract

We study the nature of the S-wave resonances N(1535) and N(1650) concerning their couplings with the ηN channel. These resonances are usually interpreted as spatially excited states of constituent quarks with the intrinsic spin 1 2 and 3 2 . In this paper, we introduce an N ∗-isobar model for the πN and ηN interactions so as to study the properties of the resonances by using the above two quark-model wave functions with pure intrinsic spin states for the isobars. The dynamical coupling of the isobars to πN and ηN channels are described by the meson-quark coupling. We find that the cooperative effect of the couplings with mesons and the tensor interaction among quarks is important for the mixing of the intrinsic spins. The physical states of the resonances are strongly mixed states of the isobars. This mixture enhances (suppresses) the coupling strength of the N(1535) (N(1650)) to ηN and reproduces the energy dependence of the π −p→ ηn reaction cross section including the peak region.