Nucleon form factors, Lorentz invariance, and nuclear photoabsorption

Abstract

The role of the intrinsic nucleon size in a phenomenological treatment of photon absorption by a nucleus is discussed. It is suggested that the complete transition matrix element should be written as a product of the purely nuclear matrix element and the intrinsic nucleon form factor. The latter is written as a function, not of $q^2$ (the four-momentum transfer), but of $q^2+{\omega }^2$, where $\omega$ is the excitation energy of the nucleus. We note that for real photons where $q^2=0\mathrm{}$ this is essentially equivalent to ${\overrightarrow{\mathrm{q}}}^2$, i.e., the three-momentum transfer.