Production of polarized vector mesons off nuclei

Using the light-cone QCD dipole formalism we investigate manifestations of color transparency (CT) and coherence length (CL) effects in electroproduction of longitudinally (L) and transversally (T) polarized vector mesons. Motivated by forthcoming data from the HERMES experiment we predict both the A and Q² dependence of the $L/T$ ratios for ρ⁰ mesons produced coherently and incoherently off nuclei. For an incoherent reaction the CT and CL effects add up and result in a monotonic A dependence of the $L/T$ ratio at different values of Q². In contrast, for a coherent process the contraction of the CL with Q² causes an effect opposite to that of CT and we expect quite a nontrivial A dependence.