Excitation of S11 resonances in pion scattering and pion photoproduction on the proton

A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. The results indicate the existence of a third and a fourth S11 resonance with the masses 1803 +/- 7 and 2117 +/- 64 MeV. In the case of pion photoproduction, we obtain background contributions to the imaginary part of the S-wave multipole which differ considerably from the result based on the K-matrix approximation. Within the dynamical model these background contributions become large and negative in the region of the S11(1535) resonance. Due to this fact much larger resonance contributions are required in order to explain the results of the recent multipole analyses. For the first S11(1535) resonance we obtain as a value of the electromagnetic helicity amplitude: A_{1/2}=72 +/- 2 (in units 10^{-3} GeV^{-1/2}). Similar values can be derived from eta photoproduction if one takes the same total width (Gamma_R=95 +/- 5 MeV) as in pion scattering and pion photoproduction