  We present the measurement of the transverse single-spin asymmetry of weak
boson production in transversely polarized proton-proton collisions at
$\sqrt{s} = 500~\text{GeV}$ by the STAR experiment at RHIC. The measured
observable is sensitive to the Sivers function, one of the transverse momentum
dependent parton distribution functions, which is predicted to have the
opposite sign in proton-proton collisions from that observed in deep inelastic
lepton-proton scattering. These data provide the first experimental
investigation of the non-universality of the Sivers function, fundamental to
our understanding of QCD.
