Measurement of the inclusive J/$$\psi $$ polarization at forward rapidity in pp collisions at $$\mathbf {\sqrt{s} = 8}$$s=8 TeV

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 \(2< p_\mathrm{T} < 15\) \(\mathrm {GeV}/c\). The three polarization parameters (\(\lambda _\theta \), \(\lambda _\varphi \), \(\lambda _{\theta \varphi }\)) are measured as a function of \(p_\mathrm{T}\) 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.