Probing ud asymmetry in the proton via quarkonium production

The sensitivity of proton-induced JΨ and ϒ production to the possible ud asymmetry in the nucleon is studied. The ratio of the quarkonium production cross sections at large xf on hydrogen over deuterium targets, σ(p + p)σ(p + d), is shown to be sensitive to this asymmetr. Predictions of various theoretical models for this ratio are presented.