We report measurements of single- and double- spin asymmetries for $W^{pm}$ and $Z/gamma^*$ boson production in longitudinally polarized $p+p$ collisions at $sqrt{s} = 510$ GeV by the STAR experiment at RHIC. The asymmetries for $W^{pm}$ were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the protons polarized quark distributions at the scale of the $W$ mass. The results are compared to theoretical predictions, constrained by recent polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.
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