The Sivers asymmetry in Drell-Yan production at the J/Ψ peak at COMPASS

The abundant production of lepton pairs via J/Ψ creation at COMPASS, π±p↑→J/ΨX→ℓ+ℓ−X, allows a measurement of the transverse single spin asymmetry, ANJ/Ψ, generated by the Sivers effect. The crucial issue of the sign change of the Sivers function in Drell-Yan lepton pair production, with respect to Semi Inclusive Deep Inelastic Scattering processes, can be addressed in a different context. Assuming that the Sivers asymmetry is related to a universal and intrinsic property of the proton, predictions for the expected magnitude of ANJ/Ψ, which turns out to be large, are given. A comparison with the suggested measurement of this single spin asymmetry - an important quantity by itself - should give valuable information.