First polarisation measurement of coherently photoproduced J/$\psi$ in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

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.