In-medium Effects in the Eta-Photoproduction through the $S_{11}$   Resonance with the Relativistic Approach
  In-medium effects in photon excitation of the S$_{11}$ resonance from nucleon and its decay to $\eta$-meson using the RMF approach is studied. We examine the in-medium effects by varing the Dirac fields of the nucleon and the S$_{11}$ resonance separately. We obtained a conclusion that the Dirac fields of the nucleon, which reduces the nucleon effective mass, enlarge the width of S$_{11}$ resonance, and small Dirac-fields of the resonance shifts the peak position, and these combined effects can explain the in-medium properties observed in the experimental data for $^{12}$C($\gamma$,$\eta$) reaction. 
Tomoyuki Maruyama and Satoshi Chiba