First measurement of quarkonium polarization in nuclear collisions at the LHC

Abstract

The polarization of inclusive J/$\psi$ and ${\rm Y}(1\mathrm{S})$ produced in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region $2.5<y<4$ and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters ${\lambda }_{\theta }$, ${\lambda }_{\varphi }$ and ${\lambda }_{\theta \varphi }$ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval $2<p_{\mathrm{T}}<10$ GeV/$c$ and $p_{\mathrm{T}}<15$ GeV/$c$ for the J/$\psi$ and ${\rm Y}(1\mathrm{S})$, respectively. The polarization parameters for the J/$\psi$ are found to be compatible with zero, within a maximum of about two standard deviations at low $p_{\mathrm{T}}$, for both reference frames and over the whole $p_{\mathrm{T}}$ range. The values are compared with the corresponding results obtained for pp collisions at $\sqrt{s}=7$ and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for ${\rm Y}(1\mathrm{S})$ production in Pb-Pb collisions are also consistent with zero.

arXiv comment21 pages, 3 captioned figures, 3 tables, authors from page 16, published version, figures at http://alice-publications.web.cern.ch/node/6223

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• Equal-time kinetic equations in a rotational field
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