Forward $J/\psi$ production in U + U collisions at $\sqrt{s_{NN}}=193$ GeV

The invariant yields, $dN/dy$, for $J/\psi$ production at forward rapidity $(1.2<|y|<2.2)$ in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=193\mathrm{GeV}$ have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor $R_{AA}$ are presented and compared with those from $\mathrm{Au}+\mathrm{Au}$ collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ collisions within the same centrality class is presented, and used to investigate the role of $c\overline{c}$ coalescence. Two different parametrizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, $N_{\mathrm{coll}}$, and these were found to give significantly different $N_{\mathrm{coll}}$ values. Results using $N_{\mathrm{coll}}$ values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the $J/\psi$ suppression, relative to binary collision scaling, is similar in $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ for peripheral and midcentral collisions, but that $J/\psi$ show less suppression for the most central $\mathrm{U}+\mathrm{U}$ collisions. The results are consistent with a picture in which, for central collisions, increase in the $J/\psi$ yield due to $c\overline{c}$ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between $c\overline{c}$ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine $N_{\mathrm{coll}}$.

Figure 1

A schematic side view of the PHENIX muon arms in 2012. The central arms, which are at midrapidity, are not shown.

Figure 2

The BBC charge distribution for $\mathrm{U}+\mathrm{U}$ collisions for the north and south detectors combined, compared with a Monte Carlo calculation using a negative binomial distribution that is fit to the data. The colored stripes show the BBC charge distribution divided into 10% wide centrality bins.

Figure 3

Dimuon invariant mass spectra at forward and backward rapidity measured in the 0–10% most central collisions [(a)–(d)] and in 60–70% midperipheral collisions [(e)–(h)], integrated over the full $p_T$ range. (a), (b), (e), and (f) show the invariant mass distribution, reconstructed from (solid symbols) same-event opposite charge-sign pairs and (open symbols) mixed-event pairs in $\mathrm{U}+\mathrm{U}$ collisions. (c), (d), (g), and (h) show the combinatorial background subtracted mass spectra. The solid line is a fit to the data using a double Gaussian line shape plus an exponential background, as described in the text.

Figure 4

Acceptance $\otimes$ efficiency as a function of collision centrality $\left(N_{\mathrm{part}}\right)$ for (squares) south arm, $-2.2<y<-1.2\mathrm{and}$ (circles) north arm, $1.2<y<2.2$. The bands represent the uncertainty due to the limited statistical precision of the embedding simulations.

Figure 5

Invariant yield of the $J/\psi$ at forward rapidity measured as a function of collision centrality $\left(N_{\mathrm{part}}\right)$ for (closed circles) $\mathrm{U}+\mathrm{U}$, (open squares) $\mathrm{Au}+\mathrm{Au}$ [16], and (open diamonds) $\mathrm{Cu}+\mathrm{Cu}$ [35].

Figure 6

The nuclear-modification factor, $R_{AA}$, measured as a function of collision centrality $\left(N_{\mathrm{part}}\right)$ for (closed circles) $J/\psi$ at forward rapidity in $\mathrm{U}+\mathrm{U}$ collisions, for which the $\mathrm{p}+\mathrm{p}$ reference data, measured at $\sqrt{s}=200\mathrm{GeV}$, has been scaled down by a factor 0.964 to account for the difference in $J/\psi$ cross section between $\sqrt{s}=200$ and 193 GeV; (open squares) $\mathrm{Au}+\mathrm{Au}$ data [16]. (a) The $\mathrm{U}+\mathrm{U} R_{AA}$ calculated using Woods-Saxon parameter set 1 [27] in the $\mathrm{U}+\mathrm{U}$ Glauber model calculation. (b) The $\mathrm{U}+\mathrm{U} R_{AA}$ calculated using parameter set 2 [28].

Figure 7

Invariant yield measured as a function of collision centrality for $J/\psi$ at forward rapidity for (closed circles) $\mathrm{U}+\mathrm{U}$ and (open squares) $\mathrm{Au}+\mathrm{Au}$ [16] collisions.

Figure 8

The relative nuclear-modification factor for $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ [Eq. (5)] as a function of collision centrality. Values were calculated using $N_{\mathrm{coll}}$ values obtained from the $\mathrm{U}+\mathrm{U}$ Glauber model calculation using Woods-Saxon parameter set 1 [27] and set 2 [28]. The values for set 1 and set 2 are slightly offset in centrality for clarity.

Figure 9

The ratio of the invariant yield for $\mathrm{U}+\mathrm{U}$ to that for $\mathrm{Au}+\mathrm{Au}$, as a function of collision centrality. The curves show how the ratio would vary with centrality if the invariant yield scaled with $N_{\mathrm{coll}}$ (dashed curve) or with ${N_{\mathrm{coll}}}^2$ (solid curve). The $N_{\mathrm{coll}}$ ratio curves obtained from the $\mathrm{U}+\mathrm{U}$ Glauber model calculation using Woods-Saxon parameter set 1 [27] are shown in blue, those using parameter set 2 [28] are shown in red. The $N_{\mathrm{coll}}$ value for $\mathrm{U}+\mathrm{U}$ is multiplied by a factor of 0.964 to account for the decrease of the charm cross section for $\mathrm{p}+\mathrm{p}$ collisions from 200 GeV to 193 GeV collision energy.