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Production of $η$ and $η'$ Mesons on Nucleons and Nuclei

The production of $η$ and $η^{\prime}$ mesons in photon- and hadron-induced reactions on free and quasi-free nucleons and on nuclei is reviewed. The extensive database on $γN \to ηN$, 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 $η^{\prime}$ or of $ηπ$ 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 $η$-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 $η^{\prime}$ production. This difference is reflected in the important final state interaction effects of the $η$ in nuclei that may even lead to this meson being "bound" in some systems. The evidence for this is reviewed for both $γA$ and $p A$ collisions. The inclusive photoproduction of $η$, $η^{\prime}$, and $ηπ$ pairs from nuclei provides further information regarding the production mechanism and the interaction of the $η$ and $η^{\prime}$ with nuclei and the $ηπ$ pairs may even allow access to low mass $ηA$ systems that are forbidden in direct single-meson photoproduction.