S11 Resonances in π and η Channels

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 1846 ± 47 and 2113 ± 70 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 analysis. For the first S₁₁(1535) resonance we obtain as a value of the electromagnetic helicity amplitude: A_1/2 = 72 ± 2 × 10^-3GeV^-1/2.