Photoproduction of eta mesons within a coupled-channels K-matrix approach

We investigate photoproduction of. mesons off protons and neutrons within a coupled-channels effective-Lagrangian method that is based on the K-matrix approach. The two-body final channels included are pi N, eta N, phi N, rho N, gamma N, K Lambda, and K Sigma. Nonresonant meson-baryon interactions are included in the model via nucleon intermediate states in the s and u channels and meson exchanges in the t-channel amplitude and the u-channel resonances. The nucleon resonances S(11)(1535), S(11)(1650), S(31)(1620), P(11)(1440), P(11)(1710), P(13)(1720), P(33)( 1232), P(33)(1600), D(13)(1520), D(13)(1700), and D(33)(1700) are included explicitly in calculations. Our model describes simultaneously the available data as well on total and differential cross sections as on beam and target asymmetries. This holds for the gamma p -> eta p reaction for photon energies ranging from very close to threshold to up to 3 GeV. The polarization observables show strong sensitivity to resonances that otherwise contribute only weakly to the total cross section. It is found that the pronounced bumplike structure seen in the excitation function of the gamma n -> eta n cross section at gamma energies around 1 GeV can be explained by the interference effects of S(11), P(11), and P(13) resonance contributions.