Transverse single spin asymmetries in Drell-Yan processes

Recently, it has been shown, contrary to previous beliefs, that the ${\mathit{k}}_{\perp }$ 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 $p^{\uparrow }\overrightarrow{p}\pi X$ and $p^{\uparrow }\overrightarrow{p}\pi X.\mathrm{}$ In such channels, however, other mechanisms, such as the “Collins effect” (a ${\mathit{k}}_{\perp }$ 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.