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HK: Hadronen und Kerne
HK 14: Electromagnetic Probes II
HK 14.1: Gruppenbericht
Montag, 16. März 1998, 17:00–17:30, B
Pion- and Photon induced reactions on the Proton up to √s=1.5 GeV in the K-matrix approach — •O. Scholten1, A.Yu. Korchin2, and R. Timmermans1 — 1Kernfysisch Versneller Instituut, 9747 AA Groningen, the Netherlands — 2also National Science Center ’Kharkov Institute of Physics and Technology’, 310108 Kharkov, Ukraine
The model of [1] for pion and photon induced reactions on the nucleon, based on the K-matrix approach, has been extended to higher energies. The model is fully relativistic, gauge invariant, crossing symmetric and obeys unitarity below the two-pion production threshold. Baryon resonances with mass upto 1.7 GeV have been included explicitely. At higher energies an explicit width is included in the propagators to account for decays outside the model space. Form factors depending on the resonance invariant mass have been introduced. Results of a consistent calculation for the three reactions, pion-scattering, pion-photoproduction and Compton scattering, will be compared to experimental cross sections and partial wave amplitudes. The extracted values for the resonance couplings will be discussed as well as results for the nucleon polarizability.
In the calculation Form factors depending on the resonance invariant mass have been introduced. these were necessary to improve the agreement with data. Some general aspects of off-shell form factors will be addressed. It will be shown that, for example, virtual Compton scattering [2] is sensitive to these off-shell form factors.
The self-energies for the delta-isobar and the nucleon have been calculated. Especially for the spin 1/2 sector in the Rarita-Schwinger propagator the self-energies are large. Due to the non-resonant nature of this sector the effects on observables are small.
[1] O. Scholten, A.Yu. Korchin, V. Pascalutsa and D. Van Neck, Phys. Lett. B384 (1996) 13.
[2] A.Yu. Korchin, O. Scholten and F. de Jong, Phys. Lett. B402 (1997) 1.