Electromagnetic Structure of the Nucleon

The electromagnetic structure of the nucleon is studied by using dispersion relation techniques. Contributions to the magnetic moments and mean square radii from the two-pion intermediate state are studied exhaustively. It is shown that the electromagnetic structure of the meson itself may play an important role here; this structure is also discussed. The two-pion state seems to account reasonably for the isotopic vector magnetic moment and magnetization mean square radius, but the charge-density radius appears to be much smaller than the currently accepted experimental value. As regards the isotopic scalar properties of the nucleon, we have studied the contributions from intermediate states with two $K$ mesons and nucleon-antinucleon pairs (more generally baryon pairs). The $K$-meson state is treated by perturbation theory and found to have a small effect. By use of an argument based on the unitarity of the $S$ matrix, it is shown that the pair contributions must be small. Certain general properties of the three-pion state, believed to be the most important contributor to isotopic scalar quantities, are discussed; but we are unable to make any quantitative statements.