Long range rapidity correlations and jet production in high energy nuclear collisions

TL;DR

This study analyzes high-$p_T$ charged di-hadron correlations in $d$+Au and central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV to dissect near-side yields into a jet-like component and a Δη-independent ridge. Using two independent analyses and background methods, the authors find a persistent ridge at large $|\Delta\eta|$ in central Au+Au, even for $p_T^{trig}>6$ GeV/$c$, with a jet-like peak whose shape resembles $p$+$p$ and $d$+Au references. The ridge spectrum is softer and bulk-like, while the jet-like spectrum remains harder, supporting a two-component picture where the ridge is linked to bulk or early-time phenomena rather than vacuum fragmentation alone. These results constrain ridge formation models and favor explanations involving early-time or initial-state dynamics, such as CGC-driven flux tubes, over some final-state energy loss or turbulent-field scenarios.

Abstract

The STAR Collaboration at RHIC presents a systematic study of high transverse momentum charged di-hadron correlations at small azimuthal pair separation \dphino, in d+Au and central Au+Au collisions at $\rts = 200$ GeV. Significant correlated yield for pairs with large longitudinal separation \deta is observed in central Au+Au, in contrast to d+Au collisions. The associated yield distribution in \detano$\times$\dphi can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d+Au collisions, and a component which is narrow in \dphi and \textcolor{black}{depends only weakly on} $\deta$, the "ridge". Using two systematically independent analyses, \textcolor{black}{finite ridge yield} is found to persist for trigger $\pt > 6$ \GeVc, indicating that it is correlated with jet production. The transverse momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range ($2 < \pt < 4 \GeVc$).

Figures (6)

• Figure 1: Charged di-hadron distribution (Eq. \ref{['eq:di-hadronDist']}) for $2\ \mathrm{GeV}/c < p_{t}^{assoc} < p_{t}^{trig}$. Upper left: central Au+Au, $3 < p_{t}^{trig} < 4$$\mathrm{GeV}/c$; Upper right: central Au+Au, $4 < p_{t}^{trig} < 6$$\mathrm{GeV}/c$; Lower left: minimum bias d+Au, $3 < p_{t}^{trig} < 4$$\mathrm{GeV}/c$ ; Lower right: minimum bias d+Au, $4 < p_{t}^{trig} < 6$$\mathrm{GeV}/c$. Note different vertical scales.
• Figure 2: $Y_{slice}\left(\hbox{$\Delta\eta$};\delta=0.3\right)$ (Eq. \ref{['eq:Yslice']}) for central Au+Au collisions, $2\ \mathrm{GeV}/c < p_{t}^{assoc} < p_{t}^{trig}$, and various $p_{t}^{trig}$ vs. $\Delta\eta$; the shaded bands represents the systematic uncertainties due to $v_2$ (not shown for $6 < p_{t}^{trig} < 10$$\mathrm{GeV}/c$). The solid and dashed line represents a constant or linear fit to $1 <$$|\Delta\eta|$$<1.8$; only shown for $3 < p_{t}^{trig} < 4$$\mathrm{GeV}/c$ (see text). Some data points are displaced horizontally for clarity.
• Figure 3: Upper panel: width of Gaussian fit to jet-like peak for Eq. \ref{['eq:NJeta']} (circles) and Eq. \ref{['eq:NJphi']} (triangles), 2 GeV/$c < p_{t}^{assoc} < p_{t}^{trig}$, as a function of $p_{t}^{trig}$ , for central Au+Au (filled) and d+Au (open). Some data points are displaced horizontally for clarity. Bottom panel: the distributions Eq. \ref{['eq:NJeta']} and Eq. \ref{['eq:NJphi']} for $4 < p_{t}^{trig} < 5$$\mathrm{GeV}/c$ and 2 GeV/$c < p_{t}^{assoc} < p_{t}^{trig}$.
• Figure 4: Ridge yield (Eq. \ref{['eq:Yridge']}) in $|\Delta\eta|<1.7$ and $2\ \mathrm{GeV}/c < p_{t}^{assoc} < p_{t}^{trig}$ as function of $p_{t}^{trig}$. Solid lines are the systematic uncertainty due to $v_2$.
• Figure 5: Projection $dN/d\Delta\phi |_{a,b}$ for 0.7 $<$$|\Delta\eta|$$<$ 1.4 (Eq. \ref{['eq:IDeta']}) in two trigger $p_t$ windows, for $2 < p_{t}^{assoc} < 4$$\mathrm{GeV}/c$. No background subtraction has been applied; note the suppressed zero on the vertical scale. The shaded band shows the fit of the function $k_1+k_2\cdot\cos(2\Delta\phi )$ to the recoil region 2 $<$$|\Delta\phi|$$<\pi$ for $4<p_{t}^{trig}<6$ GeV/c. The width of the band indicates the fitting error. The solid curve represents the background estimate using the ZYAM normalization for $4<p_{t}^{trig}<6$ GeV/c (systematic uncertainties are indicated by light shaded band).