Nucleon Resonance Excitation with Virtual Photons

The unitary isobar model MAID is used for a partial wave analysis of pion photoproduction and electroproduction data on the nucleon. In particular we have taken emphasis on the region of the Delta(1232) resonance and have separated the resonance and background amplitudes with the K-matrix approach. This leads to electromagnetic properties of the dressed Delta resonance, where all multipole amplitudes become purely imaginary and all form factors and helicity amplitudes become purely real at the K-matrix pole of W=M_Delta=1232 MeV. The REM=E2/M1 and RSM=C2/M1 ratios of the quadrupole excitation are compared to recent data analysis of different groups. The REM ratio of MAID2005 agrees very well with the data and has a linear behavior over the whole experimentally explored Q^2 region with a small positive slope that predicts a zero crossing around 3.5 GeV^2. The recent RSM data for Q^2 < 0.2 GeV^2 indicate a qualitative change in the shape of the ratio which can be explained by the impact of the Siegert theorem at pseudothreshold (Q^2=-0.086 GeV^2) in the unphysical region.Comment: 12 pages, 7 figures, Workshop Shape of Hadrons, Athens, Greece, 200

P11 Resonances with Dubna-Mainz-Taipei Dynamical Model for pi-N Scattering and Pion Electromagnetic Production

We present the results on P11 resonances obtained with Dubna-Mainz-Taipei (DMT) dynamical model for pion-nucleon scattering and pion electromagnetic production. The extracted values agree well, in general, with PDG values. One pole is found corresponding to the Roper resonance and two more resonances are definitely needed in DMT model. We further find indication for a narrow P11 resonance at around 1700 MeV with a width of around 50 MeV in both pi-N and gamma-pi reactions.Comment: Contribution to the Proceedings of NSTAR 2011 - The 8th International Workshop on the Physics of Excited Nucleons, May 17-20, 2011, Thomas Jefferson National Accelerator Facility, Newport News, Virginia US

Multipole analysis for pion photoproduction with MAID and a dynamical models

Results of new analysis of pion photoproduction data obtained with Dynamical and MAID models are presentedComment: 5 pages, 1 figure, talk given at the NSTAR2001 Workshop, Mainz, Germany, March 7-10, 200

Electromagnetic Excitation of Nucleon Resonances

Recent progress on the extraction of electromagnetic properties of nucleon resonance excitation through pion photo- and electroproduction is reviewed. Cross section data measured at MAMI, ELSA, and CEBAF are analyzed and compared to the analysis of other groups. On this basis, we derive longitudinal and transverse transition form factors for most of the four-star nucleon resonances. Furthermore, we discuss how the transition form factors can be used to obtain empirical transverse charge densities. Contour plots of the thus derived densities are shown for the Delta, Roper, S11, and D13 nucleon resonances.Comment: 30 pages, 19 figures, 9 table

The E2/M1 and C2/M1 ratios and form factors in N -> Delta transitions

A partial wave analysis of pion photoproduction has been obtained in the framework of fixed-t dispersion relations valid from threshold up to 500 MeV. 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 +- 0.1) %. For pion electroproduction recent experimental data from Mainz, Bates and JLab for Q^2 up to 4.0 (GeV/c)^2 have been analyzed with two different models, an isobar model (MAID) and a dynamical model. The E2/M1 ratios extracted with these two models show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and become positive with increasing Q^2. This is a possible indication of a very slow approach toward the pQCD region. The C2/M1 ratio near the photon point is found as R_{SM}(0)=(-6.5 +- 0.5) %. At high Q^2 the absolute value of the ratio is strongly increasing, a further indication that pQCD is not yet reached.Comment: 10 pages LaTeX including 3 figures. Talk given at the XVIIth European conference on Few-Body Problems in Physics, Evora, Portugal, 11 - 16 September 2000; to be published in Nucl.Phys.