  We report the first measurement of the parity violating single-spin
asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and
$W^{-}$ boson production in longitudinally polarized proton-proton collisions
at $\sqrt{s}=500 $GeV by the STAR experiment at RHIC. The measured asymmetries,
$A^{W^+}_{L}=-0.27\pm 0.10\;({\rm stat.})\pm 0.02\;({\rm syst.}) \pm
0.03\;({\rm norm.})$ and $A^{W^-}_{L}=0.14\pm 0.19\;({\rm stat.})\pm 0.02
\;({\rm syst.})\pm 0.01\;({\rm norm.})$, are consistent with theory
predictions, which are large and of opposite sign. These predictions are based
on polarized quark and antiquark distribution functions constrained by
polarized DIS measurements.
