First measurement of quarkonium polarization in nuclear collisions at the LHC

TL;DR

This paper reports the first measurement of quarkonium polarization in lead-lead collisions at the LHC, finding results consistent with zero polarization for J/ψ and Υ(1S), and comparing with proton-proton collision data.

Contribution

It provides the first experimental measurement of quarkonium polarization in nuclear collisions at the LHC, extending previous proton-proton studies.

Findings

J/ψ polarization is compatible with zero across the studied pT range.

Υ(1S) polarization results are also consistent with zero within uncertainties.

Results are compared with pp collision data at similar energies.

Abstract

The polarization of inclusive J/$\psi$ and $\Upsilon(1{\rm S})$ produced in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ 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 $\lambda_{\theta}$, $\lambda_{\phi}$ and $\lambda_{\theta\phi}$ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval $2<p_{\rm T}<10$ GeV/$c$ and $p_{\rm T}<15$ GeV/$c$ for the J/$\psi$ and $\Upsilon(1{\rm S})$, 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 $p_{\rm T}$, for both reference frames and over the whole $p_{\rm T}$ 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 $\Upsilon(1{\rm S})$ production in Pb-Pb collisions are also consistent with zero.