Do we have three $S_{11}$ resonances in the second resonance region?

Abstract

We review the status of the $S_{11}$ N$^*$ resonances in light of some recent theoretical and phenomenological results. Whereas the quark model predicts two such resonances around 1.6 GeV, there is considerable evidence for a third $S_{11}$ resonance in this energy range. This suggests that a $K\Sigma$ or $K\Lambda$ quasi-bound state may indeed exist below the kaon production threshold. We show that kaon production experiments, in particular $K^0$ photoproduction off nucleons, would be very sensitive to the existence of the third $S_{11}$ resonance.