Exotic decay modes of the nucleon resonance $N^*(1535)$ and the $\theta^+$ problem

Abstract

Baryon resonance state with strangeness $S = +1$ observed in systems $p K^0$ and $n K^+$ is interpreted as a result of the decay $N^{*+} \to p \eta$ of the well-known resonance $N^*(1535)$ (with spin-parity $j^P = {1/2}^-$ and mass $m \approx 1.54$ GeV) and the subsequent oscillation $\eta \rightleftarrows K^0$. It is argued that the latter is noticeably enhanced by the neighbourhood of the $\eta$ and $K^0$ levels and the gluon exchanges of the "penguin" form.