First polarisation measurement of coherently photoproduced J/ψ in ultra-peripheral Pb–Pb collisions at sNN=5.02 TeV

The first measurement of the polarisation of coherently photoproduced $J/\psi$ mesons in ultra-peripheral Pb–Pb collisions, using data at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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 $\mu {\text{b}}^{-1}$ ± 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 ${\lambda }_{\theta }$, ${\lambda }_{\phi }$, ${\lambda }_{\theta \phi }$ 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.