Abstract
The electroproduction of N ∗ resonances in the quark model has been extensively studied by Thronber, using the symmetric harmonic oscillator quark model, developed later by Faiman and Hendry. Encouraging results were obtained for small q 2, but there was a strong disagreement for q 2 ⩾ 1 GeV 2 not only for the absolute values but also for the ratio dσ dω ( N ∗) dσ dω ( elastic) . We show that this disagreement is mainly due to a bad choice of the frame where the non-relativistic transition form factors are calculated. There are physical reasons for choosing a frame near the Breit frame. We also introduce the quark form factor needed by the various other successes of the symmetric oscillator quark model (spectroscopy, strong and electromagnetic widths, differential photoproduction, elastic and diffractive processes), while Thornber considered point quarks. We discuss the origin of this quark form factor, either static charge distribution or mediating particle. Without any free parameter, we obtain good results for the ratio dσ dω ( N ∗) dσ dω ( elastic) up to large q 2 (∼ 3 GeV 2) for the four main bumps. However, when we discuss the contributions of all the established resonances ( M < 2.2 GeV) there arise problems with the radial excitations and the P 13(1870). Concerning the absolute magnitude, we think we may attribute the discrepancy between the dipole and gaussian form factors to relativistic corrections as first suggested by Licht and Pagnamenta, and Fujimura et al.
Published Version
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