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

Understand

The $J/\psi$ decay angular distributions have been measured in inelastic photoproduction in $e p$ 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 identified through their decay into muon pairs.
• The polar and azimuthal angles of the $\mu^+$ were measured in the $J/\psi$ rest frame and compared to theoretical predictions at leading and next-to-leading order in QCD.