Direct Observation of Dijets in Central $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

The STAR Collaboration at the Relativistic Heavy Ion Collider reports measurements of azimuthal correlations of high transverse momentum ($p_T$) charged hadrons in $\mathrm{Au}+\mathrm{Au}$ collisions at higher $p_T$ than reported previously. As $p_T$ is increased, a narrow, back-to-back peak emerges above the decreasing background, providing a clear dijet signal for all collision centralities studied. Using these correlations, we perform a systematic study of dijet production and suppression in nuclear collisions, providing new constraints on the mechanisms underlying partonic energy loss in dense matter.

Figure 1

Azimuthal correlation histograms of high $p_T$ charged hadron pairs for 0%–5% $\mathrm{Au}+\mathrm{Au}$ events, for various $p_T^{\mathrm{trig}}$ and $p_T^{\mathrm{assoc}}$ ranges. In the lower left panel, the yield is suppressed due to the constraint $p_T^{\mathrm{assoc}}<p_T^{\mathrm{trig}}$. All $p_T$ values in this and succeeding figures have units $\mathrm{GeV}/c$.

Figure 2

Azimuthal correlation histograms of high $p_T$ charged hadrons for $8<p_T^{\mathrm{trig}}<15\mathrm{GeV}/c$, for $d+\mathrm{Au}$, 20%–40% $\mathrm{Au}+\mathrm{Au}$, and 0%–5% $\mathrm{Au}+\mathrm{Au}$ events. $p_T^{\mathrm{assoc}}$ increases from top to bottom.

Figure 3

Centrality dependence (number of participants $N_{\mathrm{Part}}$) of near- and away-side yields in 200 GeV $d+\mathrm{Au}$ (leftmost points) and $\mathrm{Au}+\mathrm{Au}$ collisions, for $8<p_T^{\mathrm{trig}}<15\mathrm{GeV}/c$ and various $p_T^{\mathrm{assoc}}$ ranges. A semilog scale is used and data for $3<p_T^{\mathrm{assoc}}<4\mathrm{GeV}/c$ are scaled by 1.5 for clarity. The error bars are statistical. The horizontal bars for $3<p_T^{\mathrm{assoc}}<4\mathrm{GeV}/c$ show the systematic uncertainty due to background subtraction; it is negligible for higher $p_T^{\mathrm{assoc}}$.

Figure 4

Upper panels: $D(z_T)$ with $8<p_T^{\mathrm{trig}}<15\mathrm{GeV}/c$, for near- (left) and away-side (right) correlations in $d+\mathrm{Au}$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The dashed and solid lines are described in text. The horizontal bars on the away side show systematic uncertainty due to background subtraction. Lower panels: Ratio of $D(z_T)$ for $\mathrm{Au}+\mathrm{Au}$ relative to $d+\mathrm{Au}$. The error bars are statistical in all panels.