Interference and nuclear medium effects on the eta-mesic nuclear spectrum

The missing-mass spectrum obtained in a recoil-free transfer reaction p($^{27}$Al,$^{3}$He)$\pi^{-}$p'X is analyzed. We find that the observed peak structure arises from the coherent contributions from two reaction processes in the energy region corresponding to a bound eta ($\eta$) meson. In one of the processes the intermediate $\eta$ is captured by the nucleus to form the $\eta$-mesic nucleus $^{25}$Mg$_{\eta}$. In the other process, the $\eta$ does not form $\eta$-nucleus bound state. The interference between these two processes has caused the peak of the spectrum to appear at an $\eta$ binding energy stronger than the actual one. Our analysis also indicates that the data are consistent with an attractive N*(1535)-nucleus interaction at energies below the $\eta$N threshold.