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

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.