Electromagnetic Structure of the Nucleon with Many-Particle Structure

Abstract

The electric and magnetic form factors of nucleons are treated on the basis of the model in which the nucleon has a “central part” with spin one half, which is composed of three urbaryons (like quarks) and whose extent is of the order of magnitude of the nucleon Compton wave length. These quarks need not have a “particle-nature” in all respects, but may be some sort of quasi-particles. In the case where the quarks are fermions or parafermions, the general forms of the nonrelativistic wave functions of the three-body systems are given and the Fourier representations of the completely symmetric or anti-symmetric space wave functions are obtained in order to get the electric and magnetic form factors coming from the “central part”. The effects of the boson cloud are taken into account as the contribution from boson resonances. It has become clear that the electric form factor of proton evaluated only from the boson resonances does not agree with the recent experimental data. It is shown that if the contribution of the “central part” is taken into account, the agreement of the theoretical form factors with experiment is much improved. Although this does not necessarily establish the existence of the “central part”, it indicates at least that our model is consistent with the experimental data. The existence of the “central part” not only gives the electric and magnetic form factors by itself, but also, as the source of bosons, explains the effective masses of the boson resonances, which are smaller than the real masses required by the experimental data. If the radius of the “central” part is assumed to be much smaller than the order of magnitude of the nucleon Compton wave length, for example one-tenth of it, it is hard to get agreement with the present experimental data. In addition to this, if the absorption effect is taken into account, in the case of peripheral approximation, the assumed order of magnitude of the radius of the “central” part seems to be consistent with experiment. The existence of the “central part” suppresses the processes with impact parameter smaller than the nucleon Compton wave length. Further, since the many-particle exchange processes at relatively low energies are suppressed more severely as the number of exchanged particles increases, as a result of the existence of the “central part”, this gives one of the reasons for the apparent partial success of the peripheral approximation.