Starting from a unitary effective Lagrangian model for the meson-nucleon scattering developed in T. Feuster and U. Mosel, Phys. Rev. 58, 457 (1998), we come to a unified description of both meson scattering and photon-induced reactions on the nucleon. To this end the photon is added perturbatively, yielding both Compton scattering and meson photoproduction amplitudes. In a simultaneous fit to all available data the parameters of the nucleon resonances are extracted. We find that a global fit to the data of the various channels involving the final states $\ensuremath{\gamma}N,$ $\ensuremath{\pi}N,$ $\ensuremath{\pi}\ensuremath{\pi}N,$ $\ensuremath{\eta}N,$ and $K\ensuremath{\Lambda}$ is possible. Especially in eta photoproduction a readjustment of the masses and widths found in the fits to hadronic reactions alone is necessary to describe the data. Only for the ${D}_{13}(1520)$ do we find a possible disagreement for the helicity couplings extracted using the combined data set and pion photoproduction multipoles alone. The model dependence introduced by the restoration of gauge invariance is discussed and found to be significant mainly for resonances with small helicity couplings.

Photon- and meson-induced reactions on the nucleon

Determination of resonance couplings and high energy amplitudes in γN --> πN using fixed-t dispersion relations

We consider the impact of two recent ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\eta}n$ measurements on the $\ensuremath{\eta}N$ scattering length and $\ensuremath{\eta}N$ branching fractions for the $N(1535)$ and $N(1520)$ resonances within a coupled-channel analysis of $\ensuremath{\pi}N$ elastic scattering and $\ensuremath{\eta}N$ production data. The sensitivity of these results to model input is also explored.

Low-energy {eta}N interactions: Scattering lengths and resonance parameters

Electroproduction of η-meson at the S11 (1535) resonance

Differential cross sections of the reaction γp→ηp have been measured by using a tagged photon beam in the energy range from 808 MeV to 1008 MeV at θ * η =45°C, 80°C, 100°C and 110°C. Photon couplings of nucleon resonances with masses below 1700 MeV are determined by a partial wave analysis of the reaction γp→ηp based on the isobar model. Results are compared with photon-couplings obtained by the analysis of the reaction γN→πN and by the quark model calculation.

G. Sauvage

Papers

π° photoproduction on proton at backward angles between 700 MeV and 1150 MeV

Photoproduction of η meson between threshold and 900 MeV