$\eta^\prime$ meson under partial restoration of chiral symmetry in   nuclear medium
  In-medium modification of the eta' mass is discussed in the context of partial restoration of chiral symmetry in nuclear medium. We emphasize that the U_A(1) anomaly effects causes the eta'-eta mass difference necessarily through the chiral symmetry breaking. As a consequence, the eta' mass is expected to be reduced by order of 100 MeV in nuclear matter where about 30% reduction of chiral symmetry takes place. The strong attraction relating to the eta' mass generation eventually implies that there should be also a strong attractive interaction in the scalar channel of the eta'-N two-body system. We find that the attraction can be strong enough to form a bound state. 
Daisuke Jido, Shuntaro Sakai, Hideko Nagahiro, Satoru Hirenzaki,   Natsumi Ikeno
nucl-th hep-ph nucl-ex