Production of and mesons on nucleons and nuclei

The production of η and η ′ mesons in photon- and hadron-induced reactions on free and quasi-free nucleons and on nuclei is reviewed. The extensive database on γ N → η N , for both proton and neutron targets, is described in detail and its implications for the search for N ⋆ resonances much heavier than the dominant S 11 ( 1535 ) discussed. Though less is currently known about the production of the η ′ or of η π 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. oduction of the η -meson in p p and p n 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 ′ 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 γ A and p A collisions. The inclusive photoproduction of η , η ′ , and η π pairs from nuclei provides further information regarding the production mechanism and the interaction of the η and η ′ with nuclei and the η π pairs may even allow access to low mass η A systems that are forbidden in direct single-meson photoproduction.