Nuclear bound state of eta'(958) and partial restoration of chiral   symmetry in the eta' mass
  The in-medium mass of the $\eta^{\prime}$ meson is discussed in a context of partial restoration of chiral symmetry in nuclear medium. The $\eta^{\prime}$ mass is expected to be reduced by order of 100 MeV at the saturation density. The reduction is a consequence of the suppression of the anomaly effect on the $\eta^{\prime}$ mass induced by partial restoration of chiral symmetry. This strong attraction in $\eta^{\prime}$ nuclear systems does not accompany large absorption of $\eta^{\prime}$ into nuclear matter. This leads to the possibility of so narrow bound states of the $\eta^{\prime}$ meson in nuclei to be seen in hadronic reactions with light nuclear targets. 
Daisuke Jido, Hideko Nagahiro, Satoru Hirenzaki
nucl-th