  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 proton's 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 $0.05<x<0.2$.
