First measurement of quarkonium polarization in nuclear collisions at the LHC

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.


Phys. Lett. B 815 (2021) 136146
HEP Data
e-Print: arXiv:2005.11128 | PDF | inSPIRE

Figure 1

Examples of fits to the invariant-mass distributions in the helicity reference frame. The left plot corresponds to the J/$\psi$ mass region, while on the right a fit to the $\Upsilon(1S)$ mass region is shown. The fits are performed using an extended Crystal Ball function for the resonance signals, while the background is parameterized with a variable width Gaussian.

Figure 2

Fit to the $\jpsi$ 2D angular distributions in the helicity reference frame projected along $\cos\theta$ (left) and $\phi$ (right) for $2.5< y and="" gev="" the="" displayed="" uncertainties="" are="" statistical.="">< /y>

Figure 3

Inclusive $\jpsi$ polarization parameters as a function of transverse momentum for Pb--Pb collisions at $\ensuremath{\sqrt{s_{_{\mathrm{NN}}}}}=$ 5.02 TeV, compared with results obtained in pp collisions by ALICE at $\ensuremath{\sqrt{s}}=$ 8 TeV and by LHCb for prompt $\jpsi$ at $\ensuremath{\sqrt{s}}=$ 7 TeV (the LHCb markers were shifted horizontally by +0.3 GeV/c for better visibility) in the rapidity interval $3< y the="" error="" bars="" represent="" total="" uncertainties="" for="" pp="" results="" while="" pb--pb="" statistical="" and="" systematic="" are="" plotted="" separately="" as="" a="" vertical="" bar="" shaded="" box="" respectively.="" in="" left="" part="" of="" plot="" polarization="" parameters="" helicity="" reference="" frame="" reported="" right="" those="" collins-soper="" frame.="">< /y>