$J/\psi$ Elliptic Flow in Pb-Pb Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$

We report a precise measurement of the $J/\psi$ elliptic flow in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ with the ALICE detector at the LHC. The $J/\psi$ mesons are reconstructed at midrapidity ($|y|<0.9$) 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 the transverse momentum and centrality. A positive $v_2$ is observed in the transverse momentum range $2<p_T<8\mathrm{GeV}/c$ in the three centrality classes studied and confirms with higher statistics our earlier results at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ in semicentral collisions. At midrapidity, the $J/\psi v_2$ is investigated as a function of the transverse momentum in semicentral 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 $p_T$ 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.

Figure 1

Invariant mass distribution (top) and $⟨\cos 2(\phi -\mathrm{\Psi })⟩$ as a function of $m_{\ell \ell }$ (bottom) of opposite-sign dimuons (left) with $2<p_T<4\mathrm{GeV}/c$ and $2.5<y<4$ and dielectrons (right) with $2<p_T<6\mathrm{GeV}/c$ and $|y|<0.9$, in semicentral (20%–40%) Pb-Pb collisions.

Figure 2

Inclusive $J/\psi v_2(p_T)$ at forward and midrapidity for semicentral (20%–40%) Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. Calculations from transports model by Refs. [8, 25] are also shown.

Figure 3

Inclusive $J/\psi v_2(p_T)$ at forward rapidity in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ for three centrality classes: 5%–20%, 20%–40%, and 40%–60%. The average of $D^0$, $D^{+}$, and $D^{*+}$ $v_2(p_T)$ at mid-$y$ in the centrality class 30%–50% is also shown for comparison [22].