Puzzles in eta photoproduction: the 1685~MeV narrow peak

Abstract

We claim that a narrow peak in the cross section near 1685~MeV in the $\gamma n\to \eta n$ channel can be explained through a peculiar radial behaviour of the $p$-wave quark states with $j=1/2$ and $j=3/2$ in the low lying S11 resonances and the opening of the $K\Sigma$ threshold rather than by an exotic resonance. We explain the mechanism of its formation in the framework of a coupled channel formalism which incorporates quasi-bound quark-model states corresponding to the two low lying resonances in the S11 partial wave. A relation to the Single Quark Transition Model is pointed out.