Virtual photons in theory and experiment

Before the last decade nuclear electrodisintegration experiments in the region of the giant dipole resonance were carried out primarily as an expedient experimental alternative to photodisintegration experiments. Lack of an adequate treatment of Coulomb distortion of the incident and scattered electron's wavefunction in heavy nuclei and recoil in light nuclei when the momentum of the scattered electron was not much smaller than the momentum of the recoiling residual nucleus limited the establishment of the correspondence between photo- and electrodisintegration to roughly s-d shell nuclei. Distorted-wave Born approximation calculations have solved the former problem in virtual photon analysis of (e, X) total cross-section measurements and the effects of recoil on PWBA virtual photons have been recently investigated. In principle, an inclusive (e, X) experiment completely determines the (γ, X) cross-section. Besides those matrix elements present in the photodisintegration cross-section that are associated with the transverse form factor as q->ω, other terms associated with the Coulomb, interference, and polarization terms of the (e, e'X) cross-section appear in the (e, X) cross-section. Inclusive (e, X) experiments done at NBS to test limitations of E1 virtual photon theory are described. The proliferation of (e, e'X) experiments will intensify interest in (e, X) theory and experiments, with the X arm singles serving as an important check on the internal consistency of the (e, e'X) results. (orig.)