Interaction of eta mesons with nuclei

Back in the mid-1980s, a new branch of investigation related to the interaction of eta mesons with nuclei came into existence. It started with the theoretical prediction of possible exotic states of eta mesons and nuclei bound by the strong interaction and later developed into an extensive experimental program to search for such unstable states as well as understand the underlying interaction via eta-meson producing reactions. The vast literature of experimental as well as theoretical works that studied various aspects of eta-producing reactions such as the pi+n -> eta p, pd -> He-3 eta, p Li-6 -> Be-7 eta and gamma He-3 -> eta X, to name a few, had but one objective in mind: to understand the eta-nucleon (eta N) and hence the eta-nucleus interaction which could explain the production data and confirm the existence of some eta-mesic nuclei. In spite of these efforts, there remain uncertainties in the knowledge of the eta N and hence the eta-nucleus interaction. Therefore, this review is an attempt to bind together the findings in these works and draw some global and specific conclusions which can be useful for future explorations. The N-eta scattering length (which represents the strength of the eta-nucleon interaction) using different theoretical models and analyzing the data on eta production in pion, photon and proton induced reactions was found to be spread out in a wide range, namely, 0.18 <= Re a(eta)N <= 1.03 fm and 0.16 <= Sm a(eta)N <= 0.49 fm. Theoretical searches of heavy eta-mesic nuclei based on eta-nucleus optical potentials and lighter ones based on Faddeev type few-body approaches predict the existence of several quasibound and resonant states. Although some hints of eta-mesic states such as He-3(eta) and Mg-25(eta) do exist from previous experiments, the promise of clearer signals for the existence of eta-mesic nuclei lies in the experiments to be performed at the J-PARC, MAMI and COSY facilities in the near future. This review is aimed at giving an overall status of these efforts.