to $\pi N$ pairs in coincidence with the forward proton $p$.

The Brookhaven experiment did not find a theoretically expected signal [@liu86] — a relatively narrow peak of a predicted strength in the missing mass spectrum. The team working at Los Alamos did report a preliminary evidence for a wanted peak for the $^{16}$O target but this report was not confirmed (published) since then.

It was recognized in the following that the above obtained negative or incomplete results do not necessarily mean that the predicted $\eta$-mesic nuclei do not exist. It was possible that the binding energies and especially the widths of the $\eta$ bound states were theoretically underestimated. This point of view was supported by many-body calculations [@chiang91] taking into account some effects disregarded in the first theoretical works [@haider86; @liu86], in particular — dressing, binding and collisional decays of the $S_{11}(1535)$ resonance in the dense nuclear matter. The analysis of [@chiang91] was later extended and revised [@oset02a; @oset02b; @oset02c] (in particular, dressing of mesons was also included) with the main conclusion survived that $\eta$-mesic nuclei widths are bigger than those found in [@haider86; @liu86].

The next experiment has been performed at the Lebedev Physical Institute in Moscow/Troitsk [@sokol99; 