Study of the sign change of the Sivers function from STAR collaboration W/Z production data

Recent data on the transverse single spin asymmetry AN measured by the STAR Collaboration for proton-proton to Z0 + 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 sign change of the STAR data is attempted.