Muoproduction of J/ψ and the gluon distribution of the nucleon

The production of J/ψ mesons by virtual photons has been studied in inelastic muon scattering at an incident muon energy of 200 GeV. The shape of the inelastic J/ψ data is described well by the colour-singlet model of Berger and Jones. However in common with previous experiments the observed cross sections were found to be about five times those predicted. These inelastic data have been used to measure the nucleon gluon distribution in the range 0.04≤ x < 0.36, where x is the fraction of the nucleon's momentum carried by the probed gluon. The gluon distribution data points have been fitted to the form A(1 - x)n, where A is a constant determining the overall normalisation. The power of the distribution was found to be n = 5.7 ± 1.5 (stat.) ± 1.0 (syst.). (author)