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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)$π^{-}$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 ($η$) meson. In one of the processes the intermediate $η$ is captured by the nucleus to form the $η$-mesic nucleus $^{25}$Mg$_η$. In the other process, the $η$ does not form $η$-nucleus bound state. The interference between these two processes has caused the peak of the spectrum to appear at an $η$ 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 $η$N threshold.