η- and η'-mesic nuclei and U_A(1) anomaly at finite density

Abstract: We discuss theoretically the possibility of observing the bound states of the \eta and \eta'(958) mesons in nuclei. We apply the NJL model to study the \eta and \eta' meson properties at finite density and calculate the formation cross sections of the \eta and \eta' bound states with the Green function method for (\gamma,p) reaction. We also discuss the experimental feasibility at photon facilities like SPring-8. The contributions due to the \omega meson production are also included to obtain the realistic (\gamma,p) spectra. We conclude that we can expect to observe resonance peaks in (\gamma,p) spectra for the formation of meson bound states and we can deduce new information on \eta and \eta' properties at finite density. These observations are believed to be essential to know the possible mass shift of \eta' and deduce new information on the effective restoration of the U_A(1) anomaly in the nuclear medium.