Anomalous mass number dependence of nuclear transparency

We examine the target mass number (A) dependence of the nuclear transparency calculated for (e, e′ p) reactions. Generally, the transparency is proportional to A−13 for large Aʹs and has weaker A-dependence as A decreases. The conventional nuclear physics calculation yields a transparency whose A−13 dependence is reached in rather light nuclei (around A = 20). If either the proton mean-free path in nuclei becomes substantially longer, or the internal structure of the proton is explicitly incorporated in the calculation, however, the A−13 dependence is reached only in extremely heavy nuclei. The A-dependence should thus serve as a signature of the so-called color-transparency phenomenon.