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

TL;DR

This paper measures the angular distributions of $J/$ mesons in inelastic photoproduction at HERA, comparing experimental data with QCD predictions to understand the meson production mechanism.

Contribution

It provides the first detailed measurement of $J/$ helicity distributions in inelastic photoproduction at HERA, testing QCD models at different orders.

Findings

Angular distributions agree with QCD predictions at certain energies.

Results constrain models of $J/\psi$ production mechanisms.

Data supports or challenges specific QCD calculations.

Abstract

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.