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{\rm 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.
Measured polarisation parameter in the lambda formalism for coherent J/psi in the helicity frame. The central values are given with statistical and total systematic uncertainties.
We report on the measurement of the inclusive J/$\psi$ polarization parameters in pp collisions at a center of mass energy $\sqrt{s} = 8$ TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb$^{-1}$. J/$\psi$ resonances are reconstructed in their di-muon decay channel in the rapidity interval $2.5 < y < 4.0$ and over the transverse-momentum interval $2 < p_{\rm T} < 15$ GeV/$c$. The three polarization parameters ($\lambda_\theta$, $\lambda_\varphi$, $\lambda_{\theta\varphi}$) are measured as a function of $p_{\rm T}$ both in the helicity and Collins-Soper reference frames. The measured J/$\psi$ polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at $\sqrt{s} = 7$ TeV.
ALICE inclusive J/psi polarization parameters lambda_theta in the Collins-Soper frame as a function of p_T in pp collisions at sqrt{s} = 8 TeV in the rapidity interval 2.5 < y < 4.0. The error bars represent the total uncertainties.
ALICE inclusive J/psi polarization parameters lambda_phi in the Collins-Soper frame as a function of p_T in pp collisions at sqrt{s} = 8 TeV in the rapidity interval 2.5 < y < 4.0. The error bars represent the total uncertainties.
ALICE inclusive J/psi polarization parameters lambda_thetaphi in the Collins-Soper frame as a function of p_T in pp collisions at sqrt{s} = 8 TeV in the rapidity interval 2.5 < y < 4.0. The error bars represent the total uncertainties.
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