Photoproduction of eta and etaprime mesons on the nucleon

The isobar models eta-MAID and eta'-MAID have been used to analyze new data on quasi-free eta photoproduction on the deuteron from Bonn and recent eta' data on the proton from Jlab. In eta photoproduction on the neutron a bump around W=1700 MeV was observed which could possibly arise from a narrow P11 state that is discussed as a non-strange member of the Theta^+ anti-decuplett. In eta' photoproduction on the proton resonance contributions are found that can be attributed to missing resonances in the energy region around W=1900 MeV.Comment: 9 pages, 4 figures, Workshop Meson2006, Cracow, Poland, 200

Towards a model-independent partial wave analysis for pseudoscalar meson photoproduction

Amplitude and partial wave analyses for pion, eta or kaon photoproduction are discussed in the context of `complete experiments'. It is shown that the model-independent helicity amplitudes obtained from at least 8 polarization observables including beam, target and recoil polarization can not be used to determine underlying resonance parameters. However, a truncated partial wave analysis, which theoretically requires only 5 observables will be possible with minimal model input.Comment: Proc. of NSTAR2011, 6 pages, 1 figur

Dispersion Theoretical Analysis of Pion Photoproduction at Threshold and in the Delta Region

A new partial wave analysis of pion photoproduction has been obtained in the framework of fixed-t dispersion relations valid form threshold up to 500 MeV. It is based on new Mainz data for \pi^{0} and \pi^{+} production off the proton and both older and more recent data from Bonn, Frascati and TRIUMF for \pi^{+} and \pi^{-}. At threshold we obtain a good agreement with the existing data for both charged and neutral pion production. In the resonance region we have precisely determined the electromagnetic properties of the \Delta(1232) resonance, in particular the E2/M1 ratio R_{EM}=(-2.5 \pm 0.1) %. We show that a model independent resonance background subtraction is possible with the speed-plot technique and obtain the \Delta pole at W =(1211 - 50i) MeV and the E2/M1 ratio of the residues as R_{\Delta} = - 0.035 - 0.046 i.Comment: 5 pages of LATEX, invited talk given at the 7th International Symposium on Meson-Nucleon Physics and the Structure of the Nucleon (MENU97), Vancouver, B.C., Canada, July 28 - August 1, 199

Helicity Amplitudes and Sum Rules for Real and Virtual Photons

Results of the recently developed unitary isobar model (MAID) are presented for helicity amplitudes, spin asymmetries, structure functions and relevant sum rules for real and virtual photons in the resonance region. Our evaluation of the energy-weighted integrals is in good agreement for the proton but shows big discrepancies for the neutron.Comment: 10 pages LaTeX including 3 figures, Talk given at the GDH 2000 Conference, Mainz, June 14-17, 2000; to be published in World Scientifi

Electromagnetic properties of baryon resonances

Longitudinal and transverse transition form factors for most of the four-star nucleon resonances have been obtained from high-quality cross section data and polarization observables measured at MAMI, ELSA, BATES, GRAAL and CEBAF. As an application, we further show how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown and compared for the Roper and S11 nucleon resonances.Comment: 5 pages, 3 figures, Proceedings of CIPANP 201

On the Coupling of the η\eta Meson to the Nucleon

The pseudoscalar and pseudovector $\eta N$ coupling constants are calculated from an effective vertex associated with the $a_0(980)\pi N$ triangle diagram. The predicted values are in agreement with the ones concluded from fitting $\eta$ photoproduction amplitudes. In this context we stress the importance of the properties of the scalar meson octet for $\eta$ meson physics.Comment: 11 pages LATEX and 2 postscript figures included in a self-extracting uufile type archiv

Analysis of eta production using a generalized Lee model

We have investigated the processes N($\pi$, $\pi$)N and N($\pi$, $\eta$)N close to eta threshold using a simple, nonrelativistic Lee model which has the advantage of being analytically solvable. It is then possible to study the Riemann sheets of the S-matrix and the behavior of its resonance poles especially close to threshold. A theoretical simulation of the experimental cusp effect at eta threshold leads to a characteristic distribution of poles on the Riemann sheets. We find a pole located in the $4^{th}$ Riemann sheet that up to now has not been discussed. It belongs to the cusp peak at eta threshold. In addition we obtain the surprising result using the Lee model that the resonance $S_{11}(1535)$ does not play a large role. The main features of the experimental data can be reproduced without explicitly introducing this resonance. Furthermore, we have also studied the reactions N($\gamma$, $\pi$)N and N($\gamma$, $\eta$)N and find reasonable agreement between the data and both models with and without the $S_{11}(1535)$ resonance.Comment: 17 pages LATEX including 13 Figurs, submitted to Z. Phys.

MAID Analysis Techniques

MAID is a unitary isobar model for a partial wave analysis of pion photo- and electroproduction in the resonance region. It is fitted to the world data and can give predictions for multipoles, amplitudes, cross sections and polarization observables in the energy range from pion threshold up to W=2 GeV and photon virtualities Q^2 < 5 GeV^2. Using more recent experimental results from Mainz, Bates, Bonn and JLab for Q^2 up to 4.0 GeV^2, the Q^2 dependence of the helicity couplings A_{1/2}, A_{3/2}, S_{1/2} has been extracted for a series of four star resonances. We compare single-Q^2 analyses with a superglobal fit in a new parametrization of Maid2005. Besides the (pion) MAID, at Mainz we maintain a collection of online programs for partial wave analysis of eta, eta' and kaon photo- and electroproduction which are all based on similar footings with field theoretical background and baryon excitations in Breit-Wigner form.Comment: 10 pages, 3 figurs, 2 tables, Proc. of NSTAR2005, Tallahassee, FL, US