Evidence for the Onset of Color Transparency in $\rho^0$   Electroproduction off Nuclei
  We have measured the nuclear transparency of the incoherent diffractive $A(e,e'\rho^0)$ process in $^{12}$C and $^{56}$Fe targets relative to $^2$H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced $\rho^0$'s on a nucleus relative to deuterium, which is sensitive to $\rho A$ interaction, was studied as function of the coherence length ($l_c$), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared ($Q^2$). While the transparency for both $^{12}$C and $^{56}$Fe showed no $l_c$ dependence, a significant $Q^2$ dependence was measured, which is consistent with calculations that included the color transparency effects. 
L. El Fassi, L. Zana, K. Hafidi, M. Holtrop, B. Mustapha, W. K.   Brooks, H. Hakobyan, X. Zheng, K. P. Adhikari, D. Adikaram...
nucl-ex