Where is the Baseline for Color Transparency Studies with Moderate Energy Electron Beams?

Abstract

Study of color transparency (CT) effects at moderate energies is more problematic than is usually supposed. Onset of CT can be imitated by other mechanisms, which contain no explicit QCD dynamics. In the case of the $(e,e'p)$ reaction the standard inelastic shadowing well known in the pre-QCD era, causes a substantial growth of nuclear transparency with $Q^2$ and a deviation from the Glauber model, analogous to what is assumed to be a signal of CT. In the case of exclusive virtual photoproduction of vector mesons, CT is expected to manifest itself as an increase of nuclear transparency with $Q^2$ in production of the ground states and as an abnormal nuclear enhancement for the radial excitations. We demonstrate that analogous $Q^2$-dependence can be caused at moderate energies by the variation of so called coherence length, which is an interference effect, even in the framework of the vector dominance model. One should disentangle the real and the mock CT effects in experiments planned at CEBAF, at the HERMES spectrometer, or at the future electron facility ELFE.