Physics – Nuclear Physics – Nuclear Theory
Scientific paper
2000-12-13
Nucl.Phys. A689 (2001) 205-214
Physics
Nuclear Physics
Nuclear Theory
10 pages LaTeX including 3 figures. Talk given at the XVIIth European conference on Few-Body Problems in Physics, Evora, Portu
Scientific paper
10.1016/S0375-9474(01)00835-1
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.
Drechsel Dieter
Hanstein O.
Kamalov Sabit S.
Tiator Lothar
Yang Shin Nan
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