Published: 2016
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In this paper, we present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at √s = 500 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. In conclusion, these data provide the first experimental investigation of the nonuniversality of the Sivers function, fundamental to our understanding of QCD.