  Azimuthal correlations for large transverse momentum charged hadrons have
been measured over a wide pseudo-rapidity range and full azimuth in Au+Au and
p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV. The small-angle correlations
observed in p+p collisions and at all centralities of Au+Au collisions are
characteristic of hard-scattering processes already observed in elementary
collisions. A strong back-to-back correlation exists for p+p and peripheral Au
+ Au. In contrast, the back-to-back correlations are reduced considerably in
the most central Au+Au collisions, indicating substantial interaction as the
hard-scattered partons or their fragmentation products traverse the medium.
