The Sivers asymmetry in $J/\Psi$ and lepton pair production at COMPASS

The abundant production of lepton pairs via $J/\Psi$ creation at COMPASS, $\pi ^\pm \, p^\uparrow \to J/\Psi \, X \to \ell^+ \ell^- X$, allows a measurement of the transverse Single Spin Asymmetry generated by the Sivers effect. The crucial issue of the sign change of the Sivers function in lepton pair production, with respect to Semi Inclusive Deep Inelastic Scattering processes, can be solved. Predictions for the expected magnitude of the Single Spin Asymmetry, which turns out to be large, are given.