The E2/M1 and C2/M1 ratios and form factors in N -> Delta transitions
L. Tiator, D. Drechsel, O. Hanstein, S. S. Kamalov, S. N. Yang
Abstract
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 Δ(1232) resonance, in particular the E2/M1 ratio REM=(-2.5 +- 0.1) %. For pion electroproduction recent experimental data from Mainz, Bates and JLab for Q2 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 Q2. 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 RSM(0)=(-6.5 +- 0.5) %. At high Q2 the absolute value of the ratio is strongly increasing, a further indication that pQCD is not yet reached.
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