Resonance Models for Nucleon Electromagnetic Structure

Abstract

Theoretical restrictions that must be imposed on models of nucleon electromagnetic structure are derived. The equality of electric and magnetic form factors at the threshold for nucleon-antinucleon annihilation ($q^2=-4\mathrm{}{M}^2$) is established from the proper momentum dependence of the $s$- and $d$-wave matrix elements of the electromagnetic current density. The role of this equality in approximate theoretical treatments of $G_E$ and $G_M$ is discussed. Certain general implications of recent elastic electron-proton scattering data are interpreted as additional constraints to be imposed on resonance models which have no "core contributions." One such proposed four-pole model involving the [$\omega$, $\phi$, $\rho$, $B$] vector resonances is found to be inconsistent with certain of these restrictions.