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, λθ φ) 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.