Color transparency versus quantum coherence in electroproduction of vector mesons off nuclei

So far no theoretical tool for the comprehensive description of exclusive electroproduction of vector mesons off nuclei at medium energies has been developed. We suggest a light-cone QCD formalism that is valid at any energy and incorporates formation effects (color transparency), the coherence length, and the gluon shadowing. At medium energies, color transparency (CT) and the onset of coherence length (CL) effects are not easily separated. Indeed, although nuclear transparency measured by the HERMES experiment rises with $Q^2,$ it agrees with predictions of the vector-dominance model (VDM) without any CT effects. Our new results and observations are as follows. (i) The good agreement with the VDM found earlier is accidental and related to the specific correlation between $Q^2$ and CL for HERMES kinematics; (ii) CT effects are much larger than have been estimated earlier within the two-channel approximation. They are even stronger at low energies than at high energies and can be easily identified by HERMES or at JLab; (iii) gluon shadowing, which is important at high energies, is calculated and included; (iv) our parameter-free calculations explain well available data for variation of nuclear transparency with virtuality and energy of the photon; and (v) predictions for electroproduction of ρ and $\phi$ are provided for future measurements at HERMES and JLab.