Measurement of J/ψ helicity distributions in inelastic photoproduction at HERA

The J/ψ decay angular distributions have been measured in inelastic photoproduction in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb−1. The range in photon-proton centre-of-mass energy, W, was 50 <W <180 GeV. The J/ψ mesons were identified through their decay into muon pairs. The polar and azimuthal angles of the μ+ were measured in the J/ψ rest frame and compared to theoretical predictions at leading and next-to-leading order in QCD.