Eta-Mesic Nucleus and COSY-GEM Data

Abstract

The experimental data of the COSY-GEM Collaboration for the recoil-free transfer reaction p (27Al, 3He) \pi - p' X, leading to the formation of bound state of eta (\eta) meson in 25Mg nucleus, is reanalyzed in this paper. In particular, predicted values of binding energy and half-width of the \eta -mesic nucleus 25Mg\eta, given by different theoretical approaches, are compared with the ones obtained from the experimental missing mass spectrum. It is found that the spectrum can be explained reasonably well if interference effect of another process, where \eta is not bound in 25Mg but is scattered by the nucleus and emerge as a pion, is taken into account. The data also indicate that the interaction between N*(1535) and a nucleus is attractive in nature.