Measurement of inelastic J/psi and psi^prime photoproduction at HERA

TL;DR

This paper reports precise measurements of inelastic J/psi and psi' photoproduction cross sections at HERA, comparing results with advanced QCD theoretical models to test their validity in describing charmonium production.

Contribution

It provides the first detailed measurement of the psi' to J/psi cross section ratio as a function of W, z, and p_T, and compares these with non-relativistic QCD and k_T-factorisation predictions.

Findings

Measured J/psi and psi' cross sections over broad kinematic ranges.

Found that theoretical models show varying degrees of agreement with data.

Provided constraints on QCD models of quarkonium production.

Abstract

The cross sections for inelastic photoproduction of J/psi and psi^prime mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb-1 collected in the period 1996--2007. The psi^prime to J/psi cross section ratio was measured in the range 0.55 < z < 0.9 and 60 < W < 190 GeV as a function of W, z and p_T. Here W denotes the photon-proton centre-of-mass energy, z is the fraction of the incident photon energy carried by the meson and p_T is the transverse momentum of the meson with respect to the beam axis. The J/psi cross sections were measured for 0.1 < z < 0.9, 60 < W < 240 GeV and p_T > 1 GeV. Theoretical predictions within the non-relativistic QCD framework including NLO colour--singlet and colour--octet contributions were compared to the data, as were predictions based on the k_T--factorisation approach.