$J/\psi$ suppression at forward rapidity in Au $+$ Au collisions at $\sqrt{s_{NN}}=39$ and 62.4 GeV

We present measurements of the $J/\psi$ invariant yields in $\sqrt{s_{{}_{NN}}}=39$ and 62.4 GeV Au $+$ Au collisions at forward rapidity ($1.2<\left|y\right|<2.2$). Invariant yields are presented as a function of both collision centrality and transverse momentum. Nuclear modifications are obtained for central relative to peripheral Au $+$ Au collisions ($R_{\mathrm{CP}}$) and for various centrality selections in Au $+$ Au relative to scaled $p$ $+$ $p$ cross sections obtained from other measurements ($R_{\mathrm{AA}}$). The observed suppression patterns at 39 and 62.4 GeV are quite similar to those previously measured at 200 GeV. This similar suppression presents a challenge to theoretical models that contain various competing mechanisms with different energy dependencies, some of which cause suppression and others enhancement.

Figure 1

The unlike-sign invariant mass distribution (blue) for all Au $+$ Au centralities and both muon arms is shown along with the uncorrelated background calculation from mixed event pairs (red) for 62 GeV (top two panels) and 39 GeV (bottom two panels). The lower panel in each pair shows the subtracted distribution and a fit to the data.

Figure 2

$J/\psi$ invariant yields (scaled by $1/\langle N_{\mathrm{coll}}\rangle$) are shown for Au $+$ Au collisions at 39, 62.4, and 200 GeV as a function of the number of participating nucleons. The solid error bars represent the uncorrelated point-to-point uncertainties (quadrature sum of statistical and type A); the boxes represent the correlated (type B) systematic uncertainties.

Figure 3

$J/\psi$ invariant yields in minimum bias Au $+$ Au collisions at 39 and 62.4 GeV as a function of transverse momentum. The solid error bars are the quadrature sum of the statistical and type A systematic uncertainties, and the boxes represent the correlated (type B) systematic uncertainties.

Figure 4

$J/\psi$ $R_{\mathrm{CP}}$ for 0–20$\%$, 20–40$\%$, and 40–60$\%$ (central) relative to 60–86$\%$ (peripheral) Au $+$ Au collisions at 62.4 GeV. For comparison, $R_{\mathrm{CP}}$ results from Au $+$ Au collisions at 200 GeV [4] are shown with a peripheral bin of 60–93$\%$, where the $\langle N_{\mathrm{coll}}\rangle$ value is a close match. The solid error bars are the quadrature sum of the statistical and type A systematic uncertainties, and the boxes represent the correlated (type B) systematic uncertainties.

Figure 5

$J/\psi$ $R_{\mathrm{CP}}$ for 0–40$\%$ (central) relative to 40–87$\%$ (peripheral) Au $+$ Au collisions at 39 GeV. For comparison, $R_{\mathrm{CP}}$ results from Au $+$ Au collisions at 200 GeV [4] are shown with a peripheral bin of 40–93$\%$, where the $\langle N_{\mathrm{coll}}\rangle$ value is a close match. The solid error bars are the quadrature sum of the statistical and type A systematic uncertainties, and the boxes represent the correlated (type B) systematic uncertainties.

Figure 6

$J/\psi$ $R_{\mathrm{AA}}$ at $\sqrt{s_{{}_{NN}}}$ $=$ 39, 62.4, and 200 GeV [4]. The solid error bars are the quadrature sum of the statistical and type A systematic uncertainties, and the boxes represent the correlated (type B) systematic uncertainties. The global systematic uncertainties are quoted in the legend for each energy's results.

Figure 7

The number of $J/\psi$ per produced charm pair ($\times {10}^{-3}$) in Au $+$ Au central collisions ($N_{\mathrm{part}}=360$) at midrapidity. Shown are the direct $J/\psi$ and regeneration contributions. Calculation details and figure from Ref. [30].

Figure 8

The $J/\psi$ nuclear modification factor $R_{\mathrm{AA}}$ as a function of the number of participating nucleons $N_{\mathrm{part}}$ for $\sqrt{s_{{}_{NN}}}$ $=$ 39, 62.4, 200 GeV [4] Au $+$ Au collisions. Calculation results are shown from Ref. [31] for the total $J/\psi$ $R_{\mathrm{AA}}$ and the separate contribution of direct $J/\psi$ suppression and regeneration (scaled down by $\times 0.5$ for visual clarity). The PHENIX experimental data points are shown for comparison.

Figure 9

$J/\psi$ cross section as a function of rapidity in $p$+$p$ collisions at 200 GeV. The CEM calculation is shown as a black solid line with a gray band for its uncertainty. In comparison, PHENIX measurements are shown as red points [33].

Figure 10

$J/\psi$ cross section as a function of rapidity in $p$+$p$ collisions at 39 GeV. The CEM calculation is shown as a black solid line with a gray band for its uncertainty. Data points and fit function are the result of the $p$+$A$ data (E789: [34, 35]) extrapolation to $p$+$p$ as described in the text.

Figure 11

$J/\psi$ cross section as a function of rapidity in $p$+$p$ collisions at 62.4 GeV. The CEM calculation is shown as a black solid line with a gray band for its uncertainty. Data points from ISR measurements (Antreasyan [36], Clark [37], and Kourkoumelis [38]) are shown as detailed in the text.