Observation of a Coherence Length Effect in Exclusive ${\mathit{\rho }}^0$ Electroproduction

Exclusive incoherent electroproduction of the ${\rho }^0\left(770\right)$ meson from ${}^1\mathrm{H}$, ${}^2\mathrm{H}$, ${}^3\mathrm{He}$, and ${}^{14}\mathrm{N}$ targets has been studied by the HERMES experiment at squared four-momentum transfer $Q^2>0.4\mathrm{}{\mathrm{GeV}}^2$ and positron energy loss $\nu$ from 9 to 20 GeV. The ratio of the ${}^{14}\mathrm{N}$ to ${}^1\mathrm{H}$ cross sections per nucleon, known as the nuclear transparency, was found to decrease with increasing coherence length of quark-antiquark fluctuations of the virtual photon. The data provide clear evidence of the interaction of the quark-antiquark fluctuations with the nuclear medium.