Single spin asymmetry for the Drell-Yan process

We calculated the single spin asymmetries for the reaction $P+P(\uparrow)\rightarrow l \overline{l}+X$ in the framework of twist-3 QCD for HERA energies. The necessary imaginary phase is produced by the on-shell contribution of the quark propagator, while the long distance part is analogous to that providing the direct photon asymmetry calculated by J. Qiu and G. Sterman. The asymmetry turns out to be generally of the order percent.