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 param- eters (λθ , λφ , λθ φ ) are measured as a function of pT both in the helicity and Collins-Soper reference frames. The mea- sured 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.