  We calculate the differential Drell-Yan production cross section in
proton-nucleus collisions by including both next-to-leading order perturbative
effects and effects of the nuclear medium. We demonstrate that dilepton
production in fixed target experiments is an excellent tool to study
initial-state parton energy loss in large nuclei and to accurately determine
the stopping power of cold nuclear matter. We provide theoretical predictions
for the attenuation of the Drell-Yan cross section at large values of Feynman
$x_F$ and show that for low proton beam energies experimental measurements at
Fermilab's E906 can clearly distinguish between nuclear shadowing and energy
loss effects. If confirmed by data, our results may help determine the quark
radiation length in cold nuclear matter $X_0 \sim 10^{-13}$ m.
