The first measurement of the polarisation of coherently photoproduced J$/\psi$ mesons in ultra-peripheral Pb-Pb collisions, using data at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, is presented. The J/$\psi$ meson is measured via its dimuon decay channel in the forward rapidity interval $-4.0 <~ y <~ -2.5$ using the ALICE detector at the CERN LHC. An event sample corresponding to an integrated luminosity of 750 $\mu\text{b}^{-1}$ $\pm$ 5% (syst) is analysed. Hadronic activity is highly suppressed since the interaction is mediated by a photon. The polar and azimuthal angle distributions of the decay muons are measured, and the polarisation parameters $\mathbf{\lambda_{\theta}}$, $\mathbf{\lambda_{\varphi}}$, $\mathbf{\lambda_{\theta\varphi}}$ are extracted. The analysis is carried out in the helicity frame. The results are found to be consistent with a transversely polarised J/$\psi$. These values are compared with previous measurements by the H1 and ZEUS experiments. The polarisation parameters of coherent J/$\psi$ photoproduction in Pb-Pb collisions are found to be consistent with the $s$-channel helicity conservation hypothesis.
Phys. Lett. B 865 (2025) 139466
HEP Data
e-Print: arXiv:2304.10928 | PDF | inSPIRE
CERN-EP-2023-068
Figure group
Figure 1
A fit to the raw invariant mass (a) and $p_{\rm T}$ distributions for events in the $-0.20 < \cos\theta < -0.12$ range evaluated in the helicity frame. The bottom panel (b) shows the dimuon $p_{\rm T}$ around the J/$\psi$ mass range. The displayed uncertainties are statistical. |   |
Figure 3
Projections of the coherent J/$\psi$ data points in (a) $\cos\theta$ and (b) $\varphi$, for events in the $-4 < y < -2.5$ rapidity interval. The curves are obtained by setting the lambda parameters in Eq.2 to the results of the fit to the distribution given in Table 2. The uncertainties on the data points are statistical. |   |
Figure 4
ALICE results displayed in terms of spin density matrix elements, along with statistical and systematic uncertainties. A comparison with the available lowest $Q^2$ ranges of the results from the H1 and ZEUS collaborations is also provided. H1 results [19] correspond to $Q^2 \sim 0.05$ GeV$^2$ consistent with photoproduction, while the shown ZEUS measurement [20] is for $2 < Q^2 < 5$ GeV$^2$. |  |