J/$ψ$ suppression at forward rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV (1606.08197v2)

Abstract: The inclusive J/$\psi$ production has been studied in Pn-Pb and pp collisions at the centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.02$ TeV, using the ALICE detector at the CERN LHC. The J/$\psi$ meson is reconstructed, in the centre-of-mass rapidity interval $2.5<y<4$ and in the transverse-momentum range $p_{\rm T}<12$ GeV/$c$, via its decay to a muon pair. In this Letter, we present results on the inclusive J/$\psi$ cross section in pp collisions at $\sqrt{s}=5.02$ TeV and on the nuclear modification factor $R_{\rm AA}$. The latter is presented as a function of the centrality of the collision and, for central collisions, as a function of the transverse momentum $p_{\rm T}$ of the J/$\psi$. The measured $R_{\rm AA}$ values indicate a suppression of the J/$\psi$ in nuclear collisions and are then compared to our previous results obtained in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The ratio of the $R_{\rm AA}$ values at the two energies is also computed and compared to calculations of statistical and dynamical models. The numerical value of the ratio for central events (0-10\% centrality) is $1.17 \pm 0.04 {\rm{(stat)}}\pm 0.20 {\rm{(syst)}}$. In central events, as a function of $p_{\rm T}$, a slight increase of $R_{\rm AA}$ with collision energy is visible in the region $2<p_{\rm T}<6$ GeV/$c$. Theoretical calculations provide a good description of the measurements, within uncertainties.

• The paper presents precise RAA measurements with RAA = 0.66±0.01(stat)±0.05(syst) for pT < 8 GeV/c, highlighting significant J/ψ suppression.
• It employs muon pair decay channel reconstruction in the forward rapidity region using the ALICE detector at the LHC.
• The findings support models of dynamic dissociation and regeneration, underscoring the complex interplay of QGP effects in heavy-ion collisions.

Overview of J/$\psi$ Suppression at Forward Rapidity in Pb-Pb Collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

The ALICE Collaboration presents a detailed paper on the suppression of J/$\psi$ production in Pb-Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.02$ TeV. This work builds on previous investigations of heavy quarkonium states as probes for the quark-gluon plasma (QGP), a state of matter consisting of deconfined quarks and gluons, produced in high-energy heavy-ion collisions.

Research Methodology

The analysis focuses on reconstructing the J/$\psi$ meson through its decay into muon pairs within the forward rapidity region $2.5 < y < 4$ and transverse momentum $p_{\rm T} < 12$ GeV/$c$. The paper employs the ALICE detector at the CERN Large Hadron Collider (LHC) to measure the inclusive J/$\psi$ production cross section in pp collisions and evaluate the nuclear modification factor $R_{\rm AA}$ in Pb-Pb collisions. This factor is a critical observable for understanding J/$\psi$ suppression due to QGP formation, comparing yields in heavy-ion collisions against those in pp collisions, scaled by the number of binary nucleon-nucleon interactions.

Key Findings

Results reveal a significant suppression of J/$\psi$ in Pb-Pb collisions, with an integrated $$R_{\rm AA}(p_{\rm T}<8 \text{ GeV}/c) = 0.66\pm0.01 \text{ (stat)} \pm0.05 \text{ (syst)}$$. When compared to prior lower-energy results at $\sqrt{s_{\rm NN}} = 2.76$ TeV, the $R_{\rm AA}$ increases slightly, potentially indicating more effective regeneration of J/$\psi$ mesons at the higher energy due to an increased production of charm quark pairs.

The differential $R_{\rm AA}$ as a function of collision centrality and transverse momentum demonstrates a pronounced suppression in more central collisions, particularly where the QGP density and temperature are highest. A subtle increase in $R_{\rm AA}$ for J/$\psi$ with $2 < p_{\rm T} < 6$ GeV/$c$ suggests contributions from regeneration mechanisms, a phenomenon supported by theoretical models that incorporate both statistical hadronization and recombination processes.

Theoretical Comparisons

The paper engages multiple theoretical models to interpret the results, including statistical hadronization models and transport models incorporating dynamic dissociation and regeneration of J/$\psi$ in the evolving medium. Notably, the 'co-mover' model and transport models such as TM1 provide qualitative agreements, asserting that J/$\psi$ mesons are suppressed more at higher energies due to increased QGP temperatures but are simultaneously regenerated through coalescence of charm quarks.

Implications and Future Directions

This research confirms the complex interplay between suppression and regeneration in J/$\psi$ production, highlighting the relevance of studying both low- and high-$p_{\rm T}$ ranges to fully understand quarkonium dynamics in the QGP. Future investigations could benefit from refined measurements with increased precision in both $R_{\rm AA}$ and charm production cross-sections, extending the analysis to other charmonium states like the $\psi(2S)$ and supporting comparisons with emerging theory developments.

The results also pave the way for further investigations under different collision systems and energies, enhancing the understanding of QGP properties and contributing to a more robust theoretical description of heavy-ion collisions. The ability to discern variations in suppression mechanisms across energy scales will provide critical insights into the QGP phase diagram and its evolution in extreme-confinement conditions.