Measurement of longitudinal spin asymmetries for weak boson production   in polarized proton-proton collisions at RHIC
  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 $0.05<x<0.2$. 
STAR Collaboration: L. Adamczyk, J. K. Adkins, G. Agakishiev, M. M.   Aggarwal, Z. Ahammed, I. Alekseev, J. Alford, C. D. An...
nucl-ex hep-ex