Disappearance of Back-To-Back High-$p_T$ Hadron Correlations in Central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$

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 $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. 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 $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.

Figure 1

Azimuthal distributions of same-sign and opposite-sign pairs for (a) $p+p$, (b) minimum bias $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$, and (c) background-subtracted central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. All correlation functions require a trigger particle with $4<p_T^{\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{g}}<6\mathrm{G}\mathrm{e}\mathrm{V}/c$ and associated particles with $2\mathrm{G}\mathrm{e}\mathrm{V}/c<p_T<p_T^{\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{g}}$. The curves are one or two Gaussian fits.

Figure 2

Azimuthal distributions ($0<|\Delta \eta |<1.4$, $4<p_T^{\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{g}}<6\mathrm{G}\mathrm{e}\mathrm{V}/c$) for $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions (solid circles) compared to the expected distributions $D^{\mathrm{m}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{l}}$ from Eq. (2) (open circles). Also shown is the elliptic flow contribution for each centrality (solid curve).

Figure 3

Ratio of $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $p+p$ [Eq. (3)] for small-angle (squares, $|\Delta \varphi |<0.75\mathrm{r}\mathrm{a}\mathrm{d}$) and back-to-back (circles, $|\Delta \varphi |>2.24\mathrm{r}\mathrm{a}\mathrm{d}$) azimuthal regions versus number of participating nucleons for trigger particle intervals $4<p_T^{\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{g}}<6\mathrm{G}\mathrm{e}\mathrm{V}/c$ (solid) and $3<p_T^{\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{g}}<4\mathrm{G}\mathrm{e}\mathrm{V}/c$ (hollow). The horizontal bars indicate the dominant systematic error (highly correlated among points) from the uncertainty in $v_2$.