Coupled channels approach to photo-meson production on the nucleon

The coupled channels Lagrangian approach of the Giessen model (GiM) for meson production on the nucleon is discussed and applied to a selected set of meson production channels on the nucleon, ranging from πN → πN and γN → πN, eta-production and associated strangeness production to 2πN channels in the resonance energy region. We present an updated coupled-channel analysis of eta-meson production including all recent photoproduction data on the proton. The dip structure observed in the differential cross sections at c.m. energies W=1.68 GeV is explained by destructive interference between the S₁₁(1535) and S₁₁(1560) states, not confirming the postulated sharp state. Kaon production on the nucleon is investigated in KΛ and KΣ exit channels. The approach to 2πN production has been significantly improved by using the isobar approximation with σN and πΔ₁₂₃₂ intermediate states. Three-body unitarity is maintained up to interference between the isobar subchannels. We obtain R_σN(1440)=27⁺⁴_-9% and R_πΔ(1440)=12⁺⁵_-3% for the for the σN and πΔ₁₂₃₂ decay branching ratios of N*(1440) respectively. The extracted πN inelasticities and reaction amplitudes are consistent with the results of other groups.