A new $N^{\ast }(1675)$ resonance in the $\gamma N\to \eta N$ reaction

We study the $\eta$-photoproduction focusing on the new nucleon resonance which was observed at $\sqrt{s}=1675$ MeV with a narrow decay width ($\sim 10$ MeV) in the recent GRAAL experiment. Using an effective Lagrangian approach, we compute differential cross sections for the $\eta$-photoproduction. In addition to $N^{\ast }(1675)$, we employ three other nucleon resonances, i.e. $N^{\ast }(1535)$, $N^{\ast }(1650)$ and $N^{\ast }(1710)$, and vector meson exchanges which are the most relevant ones to this reaction process. As a result, we can reproduce the GRAAL data qualitatively well and observe obvious isospin asymmetry between the transition magnetic moments of $N^{\ast }(1675)$: $\mu_{\gamma nn^}\gg\mu_{\gamma pp^}$.