Search

The K-matrix approach with effective Lagrangians is used to describe the S and P pion-nucleon partial-wave amplitudes in the energy range E lab≤ 1 GeV. It is demonstrated, that treating the resonance as K-matrix a pole gives the natural way to separate the resonance and non-resonance parts of the πN amplitude. The model includes all the four-star πN resonances, the non-resonance contributions are calculated from relevant Feynman graphs without any phenomenological form factors. Different contributions to the inelastic π− p→ηn amplitude are estimated.

The recently proposed model for the pion-nucleon interaction, based on σ, ρ, N and Δ-isobar exchanges, is investigated further. We demonstrate the validity of the unitarity prescription. We prove that the inclusion of the higher resonances has a very small effect, only on the parameters of the model. The coupling constants g π NN and g π N Δ , as deduced with the model, are found to be very close to the literature values. This is interpreted as an indication that the model should provide an excellent description of the pion-nucleon interaction at low energies. Our prediction for the π N Σ -term is found to be independent of the only free parameter of our model ( G ( V ) ρ ). A number of applications, both for the pion-nucleon and pion-nucleus interactions, are discussed.

The s-wave pion-nucleus repulsion problem may be solved by taking into account nuclear relativism within the Walecka model.

A significant additional s-wave repulsion in the pion-nucleus optical potential appears in the impulse approximation when the nuclear relativism is taken into account.

Precise data for quasi-free photoproduction of η-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5GeV. The η-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to η-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the η-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free η-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the η-neutron pairs. Position and width of the peak in the invariant-mass spectrum are W ≈ 1665 MeV and FWHM Γ ≈ 25 MeV. The data are compared to the results of different models.

It is shown that the single-particle mechanism gives a good qualitative description of deep inelastic scattering on nuclei (the EMC effect) at 0.2⩽x⩽0.6.