  We derive a quantum mechanical upper bound on the amount of radiative energy
loss suffered by high energy quarks and gluons in nuclear matter. The bound
shows that the nuclear suppression observed in quarkonium production at high
$x_F$ cannot be explained in terms of energy loss of the initial or final
parton states. We also argue that no nuclear suppression is expected in the
photoproduction of light hadrons at large $x_F$.

High Energy Physics - Phenomenology