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

The J/Psi 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/Psi mesons were identied through their decay into muon pairs. The polar and azimuthal 
angles of the muon+ were measured in the J/Psi rest frame and compared to theoretical 
predictions at leading and next-to-leading order in QCD.