Elastic J/Psi Production at HERA

TL;DR

This paper reports on measurements of elastic J/Psi meson production at HERA, analyzing cross sections, energy dependence, and angular distributions to test perturbative QCD models and probe the proton's gluon density at low Bjorken x.

Contribution

It provides new experimental data on elastic J/Psi production over a wide energy and Q^2 range, testing QCD predictions and gluon density models.

Findings

Cross sections are well described by perturbative QCD models.

The energy dependence of cross sections shows little change with Q^2.

Angular distributions are consistent with s-channel helicity conservation.

Abstract

Cross sections for elastic production of J/Psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^{-1}. Results are presented for photon virtualities Q^2 up to 80 GeV^2. The dependence on the photon-proton centre of mass energy W_{gamma p} is analysed in the range 40 < \Wgp < 305 GeV in photoproduction and 40 < \Wgp < 160 GeV in electroproduction. The \Wgp dependences of the cross sections do not change significantly with Q^2 and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q^2. Differential cross sections d\sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV^2 as functions of \Wgp and Q^2. Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/Psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q^2 and is found to be described by perturbative QCD based models.