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Accessing the Sivers TMD function in proton+proton collisions through themeasurement of transverse single spin asymmetries (TSSAs) in Drell-Yan and weakboson production is an effective path to test the fundamental QCD prediction ofthe non-universality of the Sivers function. Furthermore, it provides data tostudy the spin-flavor structure of valence and sea quarks inside the proton andto test the evolution of parton distributions. The TSSA amplitude, $A_{N}$, hasbeen measured at STAR in proton+proton collisions at $\sqrt{s} = 500$ GeV, witha recorded integrated luminosity of 25 pb$^{-1}$. Within relatively largestatistical uncertainties, the current data favor theoretical models thatinclude a change of sign for the Sivers function relative to observations inSIDIS measurements, if TMD evolution effects are small. RHIC plans to runproton+proton collisions of transversely polarized beams at $\sqrt{s} = 510$GeV in 2017, delivering an integrated luminosity of 400 pb$^{-1}$. This willallow STAR to perform a precise measurement of TSSAs in both Drell-Yan and weakboson production. The present status and future plans for the Sivers functionprogram at STAR will be discussed as well as other observables sensitive to thenon-universality of the Sivers function in the Twist-3 framework, e.g. the TSSAof direct photons.

Transverse single-spin asymmetry of weak bosons and Drell-Yan production in p+p collisions at STAR: present and future