Measurement of the inclusive J/ψ polarization at forward rapidity in pp collisions at √s = 8 TeV

Abstract

We report on the measurement of the inclusive J/ ψ polarization parameters in pp collisions at a center of mass energy 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/ ψ 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<pT<15 GeV/c . The three polarization parameters ( λθ , λφ , λθφ ) are measured as a function of pT both in the helicity and Collins-Soper reference frames. The measured J/ ψ 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 s√=7 TeV.