$J∕\psi$ production in $\text{Au-Au}$ collisions at $\sqrt{s_{NN}}=200\text{GeV}$

First results on charm quarkonia production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) are presented. The yield of $J∕\psi$’s measured in the PHENIX experiment via electron-positron decay pairs at midrapidity for $\text{Au-Au}$ reactions at $\sqrt{s_{NN}}=200\text{GeV}$ is analyzed as a function of collision centrality. For this analysis we have studied $49.3\times {10}^6$ minimum bias $\text{Au-Au}$ reactions. We present the $J∕\psi$ invariant yield $dN∕dy$ for peripheral and midcentral reactions. For the most central collisions where we observe no signal above background, we quote $90\%$ confidence level upper limits. We compare these results with our $J∕\psi$ measurement from proton-proton reactions at the same energy. We find that our measurements are not consistent with models that predict strong enhancement relative to binary collision scaling.

Figure 1

(Color online) Dielectron invariant mass distribution in Au-Au reactions (top row, most central, $0–20\%$ central; middle row, midcentral, $20–40\%$ central; and bottom row, peripheral, $40–90\%$ central) for unlike sign pairs containing signal+background (left column), like sign pairs containing only background (center column), and the subtracted difference (right column).

Figure 2

(Color online) The Poisson statistical likelihood distribution as a function of the expected net signal. The distributions are for the midcentral case of $N_{\mathit{unlike}}=16$ and $N_{\mathit{like}}=8$. The dashed curve is the likelihood distribution, and the black is after eliminating the unphysical net signal less than zero and renormalizing. Vertical lines are shown to indicate the most likely value (8), the $68\%$ confidence interval values, and the $90\%$ confidence level upper limit.

Figure 3

The PHENIX $J∕\psi$ acceptance $\times$ efficiency as a function of the $J∕\psi$ transverse momentum is shown. Most of the acceptance is with one lepton into each of the two PHENIX central spectrometers. This contribution peaks at $p_T=0$ and decreases with increasing $p_T$. The rise in the acceptance at high $p_T$ is from contributions where both electron and positron are accepted into one of the PHENIX central spectrometers.

Figure 4

The $J∕\psi$ invariant yield per binary collision is shown for proton-proton reactions and three exclusive centrality ranges of Au-Au reactions all at $\sqrt{s_{NN}}=200\text{GeV}$. For the proton-proton reactions, we show the most likely value as a data point (square), the statistical error, and the estimated systematic errors as brackets. For the three Au-Au data points, we show as arrows the $90\%$ confidence level upper limits. The bracket above the limit includes the estimated systematic error on these limits. In the case of the peripheral and midcentral ranges, we also show, as a square marker, the statistically most likely value and as two horizontal dashes the $68\%$ confidence interval. The gray band indicates binary scaling and the width is the quadrature sum of the statistical and systematic error on our proton-proton data point. For the Au-Au points, the systematic error in the invariant yield per binary collision does not include the systematic error in the expected number of binary collisions. This error contribution is negligible for the central and midcentral categories and would increase the systematic error for the peripheral category by $6\%$.

Figure 5

(Color online) The $J∕\psi$ invariant yield per binary collision is shown from proton-proton reactions and three exclusive centrality ranges of $\text{Au-Au}$ reactions all at $\sqrt{s_{NN}}=200\text{GeV}$. The lines are the theoretical expectations from “normal” nuclear absorption with ${\sigma }_{c\overline{c}-N}=4.4\text{mb}$ (solid curve) and $7.1\text{mb}$ (dashed curve) cross section. The stars are the $J∕\psi$ per binary collision measured by the NA50 experiment at lower collision energy. In order to compare the shapes of the distribution, we have normalized the NA50 data to match the central value for our proton-proton results.

Figure 6

(Color online) The $J∕\psi$ invariant yield per binary collision is shown from proton-proton reactions and three exclusive centrality ranges of Au-Au reactions all at $\sqrt{s_{NN}}=200\text{GeV}$. The lowest curve is a calculation including “normal” nuclear absorption in addition to substantial absorption in a high-temperature quark-gluon plasma [16]. The curve above this is including backward reactions that recreate $J∕\psi$. The statistical model [17] result is shown as a dotted curve for midcentral to central collisions just above that. The four highest dashed curves are from the plasma coalescence model [15] for a temperature parameter of $T=400\text{MeV}$ and charm rapidity widths of $\Delta y=1.0,2.0,3.0,4.0$, from the highest to the lowest curve, respectively.