In-Medium Effects in Eta Photo-Production through the S11 Resonance in the Relativistic Approach

In-medium effects in photon excitation of the S$_{11}$ resonance from nucleonand its decay to $\eta$-meson using the RMF approach is studied. We examine thein-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 thenucleon, which reduces the nucleon effective mass, enlarge the width ofS$_{11}$ resonance, and small Dirac-fields of the resonance shifts the peakposition, and these combined effects can explain the in-medium propertiesobserved in the experimental data for $^{12}$C($\gamma$,$\eta$) reaction.Comment: 10 pages and 3 figure