We have measured the nuclear transparency of the $A(e,e^{\prime }{\pi }^{+})$ process in ${}^2\mathrm{H}$, ${}^{12}\mathrm{C}$, ${}^{27}\mathrm{Al}$, ${}^{63}\mathrm{Cu}$, and ${}^{197}\mathrm{Au}$ targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range $Q^2=1.1$ to $4.7(\mathrm{GeV}/c{)}^2$. The nuclear transparency was extracted as the super-ratio of (${\sigma }_A/{\sigma }_H$) from data to a model of pion-electroproduction from nuclei without $\pi \mathrm{\text{−}}N$ final-state interactions. The $Q^2$ and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

• Received 10 July 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.242502