We discuss theoretically the possibility of observing the bound states of the η and ${\eta }^{\text{'}}(958)$ mesons in nuclei. We apply the NJL model to study the η and ${\eta }^{\text{'}}$ meson properties at finite density and calculate the formation cross sections of the η and ${\eta }^{\text{'}}$ 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 ω 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 η and ${\eta }^{\text{'}}$ properties at finite density. These observations are believed to be essential to know the possible mass shift of ${\eta }^{\text{'}}$ and deduce new information on the effective restoration of the $U_A(1)$ anomaly in the nuclear medium.

• Received 26 June 2006

DOI:https://doi.org/10.1103/PhysRevC.74.045203