Electromagnetic form factors of the $\Delta(1232)$ excitation

Abstract

In the resonance region we have precisely determined the electromagnetic properties of the $\Delta(1232)$ -resonance, in particular the E2/M1 ratio $R_{EM}=(-2.5 \pm 0.1) \%$ . For pion electroproduction recent experimental data from Mainz, Bates, Bonn and JLab for Q2 up to 4.0 (GeV/c)2 have been analyzed with the isobar model MAID. The extracted E2/M1 ratio shows, starting from a small and negative value at the real photon point, a clear tendency to cross zero, and becomes 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 $R_{SM}(0)=(-6.5 \pm 0.5) \%$ . At high Q2, the absolute value of the ratio is strongly increasing, a further indication that pQCD is not yet reached. The electromagnetic-transition form factors of the $\Delta(1232)$ excitation are parameterized and fitted to the electroproduction data. This also shows a zero-crossing of the electric form factor G E * at $Q^2=3.6\pm 0.5$ (GeV/c)2.