Recent data on the transverse single spin asymmetry $A_N$ measured by the STAR Collaboration for $p^uparrow , p to W^pm/Z^0 , X$ reactions at RHIC allow the first investigation of the Sivers function in Drell-Yan processes and of its expected sign change with respect to SIDIS processes. A new extraction of the Sivers functions from the latest SIDIS data is performed and a critical assessment of the significance of the STAR data is attempted.
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