Measurement of the J/$\psi$ polarization with respect to the event plane in Pb-Pb collisions at the LHC

Abstract

We study the polarization of inclusive J/$\psi$ produced in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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 ($2<p_{\mathrm{T}}<4$, $4<p_{\mathrm{T}}<6$, $6<p_{\mathrm{T}}<10$ GeV/$c$) and as a function of the centrality of the collision for $2<p_{\mathrm{T}}<6$ 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 $p_{\mathrm{T}}$ and for intermediate centrality values. The polarization could be connected with the behaviour 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.

17 pages, 2 captioned figures, 1 table, authors from page 12, published version, figures at http://alice-publications.web.cern.ch/node/7845

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