Centrality, rapidity and transverse momentum dependence of $J/ψ$ suppression in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV

TL;DR

This study measures the inclusive $J/\psi$ nuclear modification factor $R_{AA}$ in Pb–Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV with ALICE, using both mid-rapidity ($|y|<0.8$) in $e^{+}e^{-}$ and forward rapidity ($2.5<y<4$) in $\mu^{+}\mu^{-}$ channels to cover the full $p_{T}$ range down to zero. The results show substantial suppression at forward rapidity with a weak centrality dependence for $\langle N_{part}\rangle>70$, and a less pronounced but nonzero suppression at mid-rapidity. A strong $p_{T}$ dependence at forward rapidity, with $R_{AA}$ decreasing from low to high $p_{T}$, together with comparisons to RHIC results, points to a significant contribution from $(re)$combination of charm quarks in the QGP. The findings reinforce the interpretation that charm-quark coalescence contributes to $J/\psi$ production at LHC energies, and they motivate dedicated $p$+Pb measurements to disentangle cold nuclear matter effects from hot-medium regeneration.

Abstract

The inclusive $J/ψ$ nuclear modification factor $R_{\rm AA}$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV has been measured by ALICE as a function of centrality in the e$^+$e$^-$ decay channel at mid-rapidity $|y|<0.8$ and as a function of centrality, transverse momentum and rapidity in the $μ^{+}μ^{-}$ decay channel at forward-rapidity $2.5<y<4$.The $J/ψ$ yields measured in Pb-Pb are suppressed compared to those in pp collisions scaled by the number of binary collisions. The $R_{\rm AA}$ integrated over a centrality range corresponding to 90% of the inelastic Pb-Pb cross section is $0.72\pm0.06$ (stat.) $\pm0.10$ (syst.) at mid-rapidity and $0.57 \pm 0.01$ (stat.) $\pm0.09$ (syst.) at forward-rapidity. At low transverse momentum, significantly larger values of $R_{\rm AA}$ are measured at forward-rapidity compared to measurements at lower energy. These features suggest that a contribution to the $J/ψ$ yield originates from charm quarks (re)combination in the deconfined partonic medium.

Figures (5)

• Figure 1: Top panels: invariant mass distributions for opposite-sign (OS) and mixed-events (ME) electron pairs. Bottom panels: OS invariant mass spectra, after the subtraction of the ME distributions, with a comparison to the Monte Carlo signal (solid lines) superimposed. The MC signal is scaled to match the integral of the OS distribution within the mass counting window. From left to right, distributions correspond to the centrality ranges 0%--10%,10%--40% and 40%--90%, respectively. The panels for the 0%--10% and 10%--40% centrality ranges are obtained from the 2011 data, while the ones for the 40%--90% centrality range are obtained from the 2010 data. The two vertical dashed lines shown in each panel indicate the mass interval used for signal counting.
• Figure 2: Top panels: fits of the dimuon invariant mass spectra in selected $p_{\mathrm{\textsc{t}}}$ intervals. Bottom panels: idem after subtraction of the combinatorial background with the event mixing technique. Distributions correspond to the centrality class 0%--90% and $2.5< y <4$.
• Figure 3: (Color online) Centrality dependence of the nuclear modification factor, $R_\mathrm{AA}$, of inclusive $\mathrm{J}/\psi$ production in $\mathrm{Pb\hbox{-}Pb}$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76\,\mathrm{TeV}$, measured at mid-rapidity and at forward-rapidity. The point to point uncorrelated systematic uncertainties (type II) are represented as boxes around the data points, while the statistical ones are shown as vertical bars. Global correlated systematic uncertainties (type I) are quoted directly in the legend.
• Figure 4: (Color online) Top panel: transverse momentum dependence of the centrality integrated $\mathrm{J}/\psi$$R_\mathrm{AA}$ measured by ALICE in $\mathrm{Pb\hbox{-}Pb}$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76\,\mathrm{TeV}$ compared to CMS Chatrchyan:2012np results at the same $\sqrt{s_{_{\mathrm{NN}}}}$. Bottom panel: transverse momentum dependence of the $\mathrm{J}/\psi$$R_\mathrm{AA}$ measured by ALICE in the 0%--20% most central $\mathrm{Pb\hbox{-}Pb}$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76\,\mathrm{TeV}$ compared to PHENIX Adare:2011yf results in the 0%--20% most central $\mathrm{Au\hbox{-}Au}$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 0.2\,\mathrm{TeV}$.
• Figure 5: (Color online) The rapidity dependence of the $\mathrm{J}/\psi$$R_\mathrm{AA}$ measured in $\mathrm{Pb\hbox{-}Pb}$ collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76\,\mathrm{TeV}$. Both mid- and forward-rapidity measurements include a common correlated systematic uncertainty of 3.4% due to $\langle T_\mathrm{AA} \rangle$. The mid-rapidity measurement covers the rapidity range $| y | < 0.8$ and the forward-rapidity one is given in intervals of 0.25 unit of rapidity from $y = 2.5$ to $y = 4$.