Abstract
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 γ N → π N , eta-production and associated strangeness production to 2π 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 Λ and K Σ exit channels. The approach to 2π N production has been significantly improved by using the isobar approximation with σN and π Δ1232 intermediate states. Three-body unitarity is maintained up to interference between the isobar subchannels. We obtain R σ N (1440)=27+4 -9 % and R π Δ (1440)=12+5 -3 % for the for the σ N and π Δ1232 decay branching ratios of N *(1440) respectively. The extracted π N inelasticities and reaction amplitudes are consistent with the results of other groups.
Highlights
The discovery of nucleon resonances in the early pionnucleon scattering experiments provided first indications for a complicated intrinsic structure of the nucleon
Excited states of the nucleon as observed in πN and γN reactions have been described in the Giessen model (GiM) coupled channels approach
The GiM approach is based on a phenomenological field theory described the baryons, mesons, and their interactions by a Lagrangian density conserving by construction the fundamental symmetries of QCD, including chiral symmetry
Summary
The discovery of nucleon resonances in the early pionnucleon scattering experiments provided first indications for a complicated intrinsic structure of the nucleon. With establishing the quark picture of hadrons and developments of the constituent quark models the interest in the study of the nucleon excitation spectra was renewed. On the theory side constituent quark (CQM) models, lattice QCD and Dyson-Schwinger approaches have been developed to describe and predict the nucleon resonance spectra. The main problem remains, a serious disagreement between the theoretical calculations and the experimentally observed baryon spectra. This concerns both the number and the properties of excited states.
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