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

The ALICE collaboration
CERN-EP-2022-066, 2022.
https://doi.org/10.17182/hepdata.139746

Abstract (data abstract)
The polarization of inclusive J/$\psi$ produced in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV at the LHC was studied by ALICE in the dimuon channel, via the measurement of the angular distribution of its decay products. The study was performed in the rapidity region $2.5<y<4$, for three transverse momentum intervals ($2<p_{\rm T}<4$, $4<p_{\rm T}<6$, $6<p_{\rm T}<10$ GeV/$c$) and as a function of the centrality of the collision for $2<p_{\rm T}<6$ GeV/c. For the first time, the polarization was 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 $p_{\rm T}$ and for intermediate centrality values. The polarization could be connected with the existence of a strong magnetic field in the early stage of quark--gluon plasma formation in Pb--Pb collisions, as well as with its behaviour as a rotating fluid with large vorticity.

  • Table 1

    Data from Figure 2, left panel

    10.17182/hepdata.139746.v1/t1

    J/$\psi$ lambda_{\theta} parameter as a function of the collision centrality, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the...

  • Table 2

    Data from Figure 2, right panel

    10.17182/hepdata.139746.v1/t2

    J/$\psi$ lambda_{\theta} parameter as a function of $\it{p}_{\rm{T}}$, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the event plane...

  • Table 3

    Data from Figure 2, right panel

    10.17182/hepdata.139746.v1/t3

    J/$\psi$ lambda_{\theta} parameter as a function of $\it{p}_{\rm{T}}$, measured for Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, in the event plane...

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