The isobar model of the γN → ηN processes in the energy range from the threshold to 1200 MeV

The isobar model of the γN → γN processes is constructed on the basis of the precision measurements of the cross section for the γp → ηp process near the threshold in Mainz (Germany) and the sensational results of recent measurements of the Σ beam asymmetry and dσ/dΩ for this process in Grenoble (France). The model involves six nucleon resonances (MeV): S ₁₁(1535), S ₁₁(1650), P ₁₁(1440), P ₁₃(1720), D ₁₃(1520), and F ₁₅(1680). The properties of these resonances are discussed. The P ₁₃(1720) and F ₁₅(1680) resonances are responsible for large positive values of the Σ beam asymmetry for the γp → ηp reaction at small angles. The contribution of the S ₁₁(1650) resonance must be taken into account in addition to the contribution of the S ₁₁(1535) resonance in order to describe the experimental dependence of the total cross section on the photon energy. The values characterizing the γn → ηn reaction are calculated on the basis of the available data on the amplitudes of electromagnetic excitation of these resonances on protons and neutrons.