Production of $\eta$ and $\eta '$ Mesons on Nucleons and Nuclei

The production of $\eta$ and $\eta^{\prime}$ mesons in photon- and hadron-induced reactions on free and quasi-free nucleons and on nuclei is reviewed. The extensive database on $\gamma N \to \eta N$, for both proton and neutron targets, is described in detail and its implications for the search for $N^{\star}$ resonances much heavier than the dominant $S_{11}(1535)$ discussed. Though less is currently known about the production of the $\eta^{\prime}$ or of $\eta\pi$ pairs, these also offer tantalizing prospects in the search for the missing isobars. The more limited data available on pion-induced production are still necessary ingredients in the partial wave analysis discussed. The production of the $\eta$-meson in $pp$ and $pn$ collisions shows once again the strong influence of the $S_{11}(1535)$ isobar, which is in contrast to the relatively weak behaviour seen near threshold for $\eta^{\prime}$ production. This difference is reflected in the important final state interaction effects of the $\eta$ in nuclei that may even lead to this meson being "bound" in some systems. The evidence for this is reviewed for both $\gamma A$ and $p A$ collisions. The inclusive photoproduction of $\eta$, $\eta^{\prime}$, and $\eta\pi$ pairs from nuclei provides further information regarding the production mechanism and the interaction of the $\eta$ and $\eta^{\prime}$ with nuclei and the $\eta\pi$ pairs may even allow access to low mass $\eta A$ systems that are forbidden in direct single-meson photoproduction.