High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at sqrt(s_NN)=200 GeV

TL;DR

A measurement of two-particle correlations with a high transverse momentum trigger particle (p(T)(trig) > 2.5 GeV/c) is presented for Au+Au collisions at square root(s(NN)) = 200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector.

Abstract

A measurement of two-particle correlations with a high transverse momentum trigger particle (p_T^{trig} > 2.5 GeV/c) is presented for Au+Au collisions at sqrt(s_NN) = 200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < Δη< 2). A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all Δη. 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 Au+Au collisions an additional correlation extended in Δηand known as the `ridge' is found to reach at least |Δη| \approx 4. The ridge yield is largely independent of Δηover the measured range, and it decreases towards more peripheral collisions. For the chosen p_T^{trig} cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

Figures (4)

• Figure 1: (color online) 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: (color online) Per-trigger correlated yield with $p_T^{trig} > 2.5$ GeV/c as a function of $\Delta\eta$ and $\Delta\phi$ for $\sqrt{s}$ and $\sqrt{s_{_{NN}}}$=200 GeV \ref{['fig:pythiacorr']} PYTHIA p+p and \ref{['fig:auaucorr']} PHOBOS 0-30% central Au+Au collisions. \ref{['fig:detaproj']} Near-side yield integrated over $|\Delta\phi| < 1$ for 0-30% Au+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: (color online) Projections of the correlated yield versus $\Delta\phi$ 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 $\Delta\phi=0$ and averaged. The dashed line is p+p PYTHIA for comparison. 90% C.L. systematic uncertainties are presented as in Fig 2c.
• Figure 4: (color online) Average near-side ($|\Delta\phi|<1$) and away-side ($\Delta\phi>1$) yields as a function of $N_{part}$ 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.