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

Excitation of S11 resonances in pion scattering and pion photoproduction on the proton

A self-consistent analysis of pion scattering and pion photoproduction within a coupled channels dynamical model is presented. In the case of pion photoproduction, we obtain background contributions to the imaginary part of the S-wave multipole which differ considerably from the result based on the K-matrix approximation. Within the dynamical model these background contributions become large and negative in the region of the S_{11}(1535) resonance. Due to this fact much larger resonance contributions are required in order to explain the results of the recent multipole analyses. For the first S_{11}(1535) resonance we obtain as a value of the dressed electromagnetic helicity amplitude: A_{1/2}=(72 +/- 2) * 10^{-3} GeV^{-1/2}. Similar values can be derived from eta photoproduction if one takes the same total width Gamma_R=95 +/- 5 MeV) as in pion scattering and pion photoproduction. The combined analysis yields considerable strength at invariant mass W>1750 MeV, which can be explained by a third and a fourth S_{11} resonance with the masses 1846 +/- 47 and 2113 +/- 70 MeV.Comment: 15 pages LATEX including 7 tables and 7 figures, revised versio

Probing Delta structure with pion electromagnetic production

The Dubna-Mainz-Taipei dynamical model for pion electromagnetic production, which can describe well the existing data from threshold up to 1 GeV photon lab energy, is presented and used to analyze the recent precision data in the $\Delta$ region. We find that, within our model, the bare Delta is almost spherical while the physical Delta is oblate. The deformation is almost saturated by the pion cloud effects. We further find that up to Q^2 = 4.0 (GeV/c)^2, the extracted helicity amplitude A_{3/2} and A_{1/2} remain comparable with each other, implying that hadronic helicity is not conserved at this range of Q^2. The ratio E_{1+}/M_{1+} obtained show, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and to become positive with increasing Q^2. This is a possible indication of a very slow approach toward the pQCD region. Finally, we find that the bare helicity amplitude A_{1/2} and S_{1/2}, but not A_{3/2}, starts exhibiting the scaling behavior at about Q^2 \ge 2.5 (GeV/c)^2.Comment: Invited talk presented at the 2nd Asia-Pacific Conference on Few-Body Problems in Physics, Shanghai, P.R. China, 2002 (10 pages LATEX including 1 table and 5 figures