Disappearance of Back-To-Back High- p T Hadron Correlations in Central A u + A u Collisions at s N N = 200     G e V

Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudorapidity range and full azimuth in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. The small-angle correlations observed in $p+p$ collisions and at all centralities of $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions are characteristic of hard-scattering processes previously observed in high-energy collisions. A strong back-to-back correlation exists for $p+p$ and peripheral $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$. In contrast, the back-to-back correlations are reduced considerably in the most central…