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Search for Quasi Bound $η$ Mesons

The search for a quasi bound $η$ meson in atomic nuclei is reviewed. This tentative state is studied theoretically as well as experimentally. The theory starts from elastic $η$ nucleon scattering which is derived from production data within some models. From this interaction the $η$ nucleus interaction is derived. Model calculations predict binding energies and widths of the quasi bound state. Another method is to derive the $η$ nucleus interaction from excitation functions of $η$ production experiments. The $s$ wave interaction is extracted from such data via final state interaction theorem. We give the derivation of $s$ wave amplitudes in partial wave expansion and in helicity amplitudes and their relation to observables. Different experiments extracting the final state interaction are discussed as are production experiments. So far only three experiments give evidence for the existence of the quasi bound state: a pion double charge exchange experiment, an effective mass measurement, and a transfer reaction at recoil free kinematics with observation of the decay of the state.