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