J/$\psi$ elliptic flow in Pb-Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}}$ = 5.02 TeV

TL;DR

This paper measures the elliptic flow of J/ψ mesons in Pb-Pb collisions at 5.02 TeV, providing insights into charm quark thermalization and medium properties in heavy-ion collisions.

Contribution

It presents the first detailed measurement of J/ψ elliptic flow at 5.02 TeV, extending previous results and comparing them with transport models to understand charm quark behavior.

Findings

Positive J/ψ v2 observed at forward rapidity in 2-8 GeV/c range

Results support charm quark thermalization in the quark-gluon plasma

Discrepancies suggest missing mechanisms in current models

Abstract

We report a precise measurement of the J/$\psi$ elliptic flow in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. The J/$\psi$ mesons are reconstructed at mid-rapidity ($|y| < 0.9$) in the dielectron decay channel and at forward rapidity ($2.5<y<4.0$) in the dimuon channel, both down to zero transverse momentum. At forward rapidity, the elliptic flow $v_2$ of the J/$\psi$ is studied as a function of transverse momentum and centrality. A positive $v_2$ is observed in the transverse momentum range $2 < p_{\rm T} < 8$ GeV/$c$ in the three centrality classes studied and confirms with higher statistics our earlier results at $\sqrt{s_{\rm NN}} = 2.76$ TeV in semi-central collisions. At mid-rapidity, the J/$\psi$ $v_2$ is investigated as a function of transverse momentum in semi-central collisions and found to be in agreement with the measurements at forward rapidity. These results are compared to transport model calculations. The comparison supports the idea that at low $p_{\rm T}$ the elliptic flow of the J/$\psi$ originates from the thermalization of charm quarks in the deconfined medium, but suggests that additional mechanisms might be missing in the models.