We study the polarization of inclusive $J/\psi$ produced in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ at the LHC in the dimuon channel, via the measurement of the angular distribution of its decay products. We perform the study in the rapidity region $2.5<y<4$, for three transverse momentum intervals ($2<p_T<4$, $4<p_T<6$, $6<p_T<10\mathrm{GeV}/c$) and as a function of the centrality of the collision for $2<p_T<6\mathrm{GeV}/c$. For the first time, the polarization is measured with respect to the event plane of the collision, by considering the angle between the positive-charge decay muon in the $J/\psi$ rest frame and the axis perpendicular to the event-plane vector in the laboratory system. A small transverse polarization is measured, with a significance reaching $3.9\sigma$ at low $p_T$ and for intermediate centrality values. The polarization could be connected with the behavior of the quark-gluon plasma, formed in Pb-Pb collisions, as a rotating fluid with large vorticity, as well as with the existence of a strong magnetic field in the early stage of its formation.

• Received 20 May 2022
• Revised 9 February 2023
• Accepted 28 March 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.042303

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