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