Production of ηη and η′η′ mesons on nucleons and nuclei

The production of ηη and η′η′ mesons in photon- and hadron-induced reactions on free and quasi-free nucleons and on nuclei is reviewed. The extensive database on γN→ηNγN→ηN, for both proton and neutron targets, is described in detail and its implications for the search for N⋆N⋆ resonances much heavier than the dominant S11(1535)S11(1535) discussed. Though less is currently known about the production of the η′η′ or of ηπηπ 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 ηη-meson in pppp and pnpn collisions shows once again the strong influence of the S11(1535)S11(1535) isobar, which is in contrast to the relatively weak behaviour seen near threshold for η′η′ production. This difference is reflected in the important final state interaction effects of the ηη in nuclei that may even lead to this meson being “bound” in some systems. The evidence for this is reviewed for both γAγA and pApA collisions. The inclusive photoproduction of ηη, η′η′, and ηπηπ pairs from nuclei provides further information regarding the production mechanism and the interaction of the ηη and η′η′ with nuclei and the ηπηπ pairs may even allow access to low mass ηAηA systems that are forbidden in direct single-meson photoproduction.