Structure and Excitation Mechanism of the N^* in Electroproduction e+p→e+N^*

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The electroproduction of the nucleon resonance ep→eN^* is theoretically investigated on the basis of the composite particle model of hadrons, in which the hadron is in a bound state of fundamental constituent particles forming a triplet. The working hypotheses or assumptions used for the structure of the nucleon and its resonances and for the electromagnetic interaction are the same as those made in a previous paper on the photoproduction γP→N^*, the predecessor of the present paper in the series of this research work: The nucleon and its resonances concerned here are described by the {56}-representation of the SU_6. In particular, the N and the N_<33> belong to the lowest {56}-representation with the S-wave inner space wave function, while other resonances are in the one-particle excitation mode. The electroproduction is assumed to be caused through the electromagnetic interaction without the exchange current among fundamental constituent particles. The selection rules are the same as those of the photoproduction, except for the Coulomb excitation. Owing to the virtuality of photon exchanged between e and p, it is expected to get more significant information from the electroproduction rather than those obtained from the photoproduction. For this reason the q^2-dependences of the differential cross section (d^2σ/dΩ´dε´), the ratio of the magnetic multipole transition to the electric one and the ratio of the Coulomb excitation to the transverse one are calculated, q being the momentum transfer conveyed by the virtual photon. The present model includes two parameters representing the linear extension of the nucleon and the boson cloud effect. By adjusting them to plausible values, it is shown that the theoretical results for the above quantities are consistent with the existing experiments, although experimental data are still scace. The characteristic feature of the theoretical results is the appearance of the zero point in (d^2σ/dΩ´dε´) vs. q^2 for the N^*(I=3/2). their positions are to be easily covered by working machines, even though the existing experiments have not covered them yet. Finally remarks are made on a possible way of determining the absolute value of the magnetic moment of the fundamental particle, and on the effect of the presence of the N^*(1688) on the observed cross section.
1967-07-25

Structure and Excitation Mechanism of the N^* in Electroproduction e+p→e+N^*

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