Medium effects to the N(1535) resonance and η mesic nuclei

The structure of $\ensuremath{\eta}$-nucleus bound systems $(\ensuremath{\eta}$ mesic nuclei) is investigated as one of the tools to study in-medium properties of the ${N(1535)(N}^{*})$ resonance by using the chiral doublet model to incorporate the medium effects of the ${N}^{*}$ resonance in a chiral symmetric way. We find that the shape and depth of the $\ensuremath{\eta}$-nucleus optical potential are strongly affected by the in-medium properties of the ${N}^{*}$ and nucleon. Especially, as a general feature of the potential, the existence of a repulsive core of the $\ensuremath{\eta}$-nucleus potential at the nuclear center with an attractive part at the nuclear surface is concluded. We calculate the level structure of bound states in this ``central-repulsive and surface-attractive'' optical potential and find that the level structure is sensitive to the in-medium properties of the ${N}^{*}.$ The ${(d,}^{3}\mathrm{He})$ spectra are also evaluated for the formation of these bound states to investigate the experimental feasibility. We also make comments on the possible existence of halo-like $\ensuremath{\eta}$ states in $\ensuremath{\beta}$-unstable halo nuclei.