Transverse single spin asymmetries in Drell-Yan processes

Recently, it has been shown, contrary to previous beliefs, that the k_⊥ distribution of quarks in a transversely polarized proton can be asymmetric. This ``Sivers effect'' had already been used to explain transverse single spin asymmetry (SSA) observed in inclusive pion production p^↑→pπX and p^↑→pπX. In such channels, however, other mechanisms, such as the ``Collins effect'' (a k_⊥ asymmetric fragmentation of a transversely polarized quark into pions), may generate SSA. The Sivers asymmetry is used here to compute the SSA in Drell-Yan processes; in this case, by considering the differential cross section in the lepton-pair invariant mass, rapidity, and transverse momentum, other mechanisms that may originate SSA cannot contribute. Estimates for BNL RHIC experiments are given.