Measurements of inelastic J/psi and psi' photoproduction at HERA

TL;DR

This paper reports measurements of inelastic J/psi and psi' photoproduction cross sections at HERA, comparing results with theoretical models including colour-singlet and colour-octet mechanisms, and analyzing helicity distributions.

Contribution

First measurements of inelastic J/psi and psi' photoproduction cross sections at HERA with detailed comparison to QCD models.

Findings

Next-to-leading-order colour-singlet model describes data well within uncertainties.

The psi'/J/psi cross-section ratio was measured for the first time in this kinematic range.

J/psi helicity distribution aligns with leading-order QCD predictions for z > 0.4.

Abstract

The cross sections for inelastic photoproduction of J/psi and psi' mesons have been measured in ep collisions with the ZEUS detector at HERA, using an integrated luminosity of 38.0 pb-1. The events were required to have 0.1 < z < 0.9 and 50 < W < 180 GeV, where z is the fraction of the incident photon energy carried by the J/psi in the proton rest frame and W is the photon-proton centre-of-mass energy. The psi' to J/psi cross-section ratio was measured in the range 0.55 < z < 0.9. The J/psi data, for various ranges of transverse momentum, are compared to theoretical models incorporating colour-singlet and colour-octet matrix elements. Predictions of a next-to-leading-order colour-singlet model give a good description of the data, although there is a large normalisation uncertainty. The J/psi helicity distribution for z > 0.4 is compared to leading-order QCD predictions.