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<p_T<15 GeV/c. The three polarization parameters ( λ_θ, λ_φ, λ_θφ ) are measured as a function of p_T 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.