Berlin 2005 – wissenschaftliches Programm
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HK: Physik der Hadronen und Kerne
HK 24: Elektromagnetische und Hadronische Proben
HK 24.5: Vortrag
Dienstag, 8. März 2005, 15:15–15:30, TU MA001
Electroproduction of Nucleon Resonances — •Lothar Tiator1, Dieter Drechsel1, Sabit Kamalov2, Mauro M Giannini3, Elena Santopinto3 und Andrea Vassallo3 — 1Institut für Kernphysik, Universität Mainz — 2Laboratory of Theoretical Physics, JINR Dubna, 14980 Moscow Region, Russland — 3Dipartimento di Fisica dell’ Universita di Genova and INFN, Sezione di Genova, Genova, Italy
The Mainz unitary isobar model MAID has been extended and used for a partial wave analysis of pion photo- and electroproduction in the resonance region W<2 GeV. Older data from the world data base and more recent experimental results from Mainz, Bates, Bonn and JLab for Q2 up to 4.0 (GeV/c)2 have been analyzed and the Q2 dependence of the helicity amplitudes have been extracted for a series of four star resonances. We compare single-Q2 analyses with a superglobal fit in a new parametrization of Maid2003 together with predictions of the hypercentral constituent quark model. As a result we find that the helicity amplitudes and transition form factors of constituent quark models should be compared with the analysis of bare resonances, where the pion cloud contributions have been subtracted. Finally, we will compare our results with recent lattice QCD calculations for the N→ Δ transition. The quenched calculations of Alexandrou et al. can describe the E/M and S/M ratios for Q2 < 1.5 GeV2 reasonably well but overestimate the dominant magnetic Gm* form factor by nearly a factor of 2 at large Q2.