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

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arxiv 2204.10171 v2 pith:QVLGFQQS submitted 2022-04-21 nucl-ex hep-ex

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

classification nucl-ex hep-ex
keywords polarizationcollisionscentralitycollisiondecayeventmeasuredmeasurement
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We study the polarization of inclusive J/$\psi$ produced in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ 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 ($2<p_{\rm T}<4$, $4<p_{\rm T}<6$, $6<p_{\rm T}<10$ 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 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 $p_{\rm T}$ 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.

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Cited by 5 Pith papers

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