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

Abstract

The production of η and η mesons in photon-and hadron-induced reactions on free and quasifree nucleons and on nuclei is reviewed. The extensive database on γN → ηN , 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 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 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 pA collisions. The inclusive photoproduction of η, η , and ηπ pairs from nuclei provides further information regarding the production mechanism and the interaction of the η and η with nuclei and the ηπ pairs may even allow access to low mass ηA systems that are forbidden in direct single-meson photoproduction.

Eta photoproduction in a combined analysis of pion- and photon-induced reactions

“…An alternative experimental window has opened in recent years with high-precision measurements of cross sections and polarization observables in eta photoproduction at photon-beam facilities like ELSA, JLab or MAMI, see Refs. [18][19][20] for reviews. For example, the first measurements from the JLab FROST target (polarization E for γp → π + n) have appeared only recently [21], and many more polarization data are expected.…”

Unresolved Issues in the Search for Eta-mesic Nuclei

“…Due to the non-availability of eta beams, the eta-nucleon (η-N) interaction can only be deduced via the eta-nucleon or eta-nucleus interaction in the final state. Fortunately, its interaction with the nucleon in the s-wave (which proceeds through the formation of an N * (1535) resonance) is attractive and leads to the possibility of forming and studying unstable bound states of eta mesons and nuclei [1] (see also [2,3,4] for an extensive list of references). After about 25 years of investigations in this field, a lot of progress has been made (1) and some evidence for the existence of such states exists, however, there still does not exist a general agreement and a final word on the strength of the eta-nucleon and eta-nucleus interaction.…”

Light Baryon Spectroscopy

“…The η meson as well as the η meson have an isospin of I = 0, which allows the exclusive study of N * resonances through the N η and N η final states. A detailed review about these final states is given in [228].…”