High Transverse Momentum Triggered Correlations over a Large Pseudorapidity Acceptance in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$

A measurement of two-particle correlations with a high transverse momentum trigger particle ($p_T^{\mathrm{trig}}>2.5\mathrm{GeV}/c$) is presented for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ over the uniquely broad longitudinal acceptance of the PHOBOS detector ($-4<\Delta \eta <2$). A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all $\Delta \eta$. As in $p+p$ collisions, the near side is characterized by a peak of correlated partners at small angle relative to the trigger particle. However, in central $\mathrm{Au}+\mathrm{Au}$ collisions an additional correlation extended in $\Delta \eta$ and known as the “ridge” is found to reach at least $|\Delta \eta |\approx 4$. The ridge yield is largely independent of $\Delta \eta$ over the measured range, and it decreases towards more peripheral collisions. For the chosen $p_T^{\mathrm{trig}}$ cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

Figure 1

Ratio of signal to background for the 10% most central collisions at long range (left) and short range (right). The estimated flow modulation [scaled by $a(\Delta \eta )$] and its uncertainty are represented by the solid line and shaded band. The uncertainty on the ZYAM parameter is represented by the width of the narrow horizontal line.

Figure 2

Per-trigger correlated yield with $p_T^{\mathrm{trig}}>2.5\mathrm{GeV}/c$ as a function of $\Delta \eta$ and $\Delta \varphi$ for $\sqrt{s}$ and $\sqrt{s_{NN}}=200\mathrm{GeV}$ (a) PYTHIA $p+p$ and (b) PHOBOS 0%–30% central $\mathrm{Au}+\mathrm{Au}$ collisions. (c) Near-side yield integrated over $|\Delta \varphi |<1$ for 0%–30% $\mathrm{Au}+\mathrm{Au}$ compared to PYTHIA $p+p$ (dashed line) as a function of $\Delta \eta$. Bands around the data points represent the uncertainty from flow subtraction. The error on the ZYAM procedure is shown as a gray band at zero. All systematic uncertainties are 90% confidence level.

Figure 3

Projections of the correlated yield versus $\Delta \varphi$ at short range (top row, $-1<\Delta \eta <1$) and long range (bottom row, $-4<\Delta \eta <-2$) for five centrality bins (most central on right). Points have been reflected about $\Delta \varphi =0$ and averaged. The dashed line is $p+p$ PYTHIA for comparison. 90% C.L. systematic uncertainties are presented as in Fig. 2c.

Figure 4

Average near-side ($|\Delta \varphi |<1$) and away-side ($\Delta \varphi >1$) yields as a function of $N_{\mathrm{part}}$ at both short range ($|\Delta \eta |<1$) and long range ($-4<\Delta \eta <-2$). The near-side yield at short range has the full PYTHIA jet yield subtracted from it. For clarity of presentation, the open points have been offset slightly and the away-side yields scaled down by a factor of 2. Boxes correspond to the combined 90% systematic uncertainties on the $v_2$ estimate and ZYAM procedure.