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 pi N -> pi N and gamma N -> pi N, eta-production and associated strangeness production to 2 pi N 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 S-11(1535) and S-11(1560) states, not confirming the postulated sharp state. Kaon production on the nucleon is investigated in K Lambda and K Sigma exit channels. The approach to 2 pi N production has been significantly improved by using the isobar approximation with sigma N and pi Delta 1232 intermediate states. Three-body unitarity is maintained up to interference between the isobar subchannels. We obtain R-sigma N(1440)=27(-9)(+4)% and R-pi Delta(1440)=12(-3)(+5)% for the for the sigma N and pi Delta(1232) decay branching ratios of N*(1440) respectively. The extracted pi N inelasticities and reaction amplitudes are consistent with the results of other groups.