Nuclear transparency with theγn→π−pprocess in4He

We have measured the nuclear transparency of the fundamental process $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{n}{\ensuremath{\pi}}^{\ensuremath{-}}p$ in ${}^{4}\mathrm{He}.$ These measurements were performed at Jefferson Lab in the photon energy range of 1.6--4.5 GeV and at ${\ensuremath{\theta}}_{\mathrm{cm}}^{\ensuremath{\pi}}=70\ifmmode^\circ\else\textdegree\fi{}$ and $90\ifmmode^\circ\else\textdegree\fi{}.$ These measurements are the first of their kind in the study of nuclear transparency in photoreactions. They also provide a benchmark test of Glauber calculations based on traditional models of nuclear physics. The transparency results suggest deviations from the traditional nuclear physics picture. The momentum transfer dependence of the measured nuclear transparency is consistent with Glauber calculations that include the quantum chromodynamics phenomenon of color transparency.