  Multiple parton interactions in relativistic heavy ion reactions result in
transverse momentum diffusion and medium induced non-Abelian energy loss of the
hard probes traversing cold and hot nuclear matter. A systematic study of the
interplay of nuclear effects on the $p_T \geq 2$ GeV inclusive hadron spectra
demonstrates that the competition between nuclear shadowing, multiple
scattering and jet quenching leads to distinctly different
enhancement/suppression of moderate and high-$p_T$ hadron production in $d+Au$
and $Au+Au$ collisions at RHIC. The associated increase of di-jet acoplanarity,
measured via the broadening of the back-to-back di-hadron correlation function,
provides an additional experimental tool to test the difference in the
dynamical properties of the media created in such reactions.
