  We present an analytic next-to-leading order QCD calculation of the partonic
cross sections for single-inclusive lepton production in hadronic collisions,
when the lepton originates from the decay of an intermediate electroweak boson
and is produced at high transverse momentum. In particular, we consider the
case of incoming longitudinally polarized protons for which parity-violating
single-spin asymmetries arise that are exploited in the $W$ boson program at
RHIC to constrain the proton's helicity parton distributions. Our calculation
enables a very fast and efficient numerical computation of the relevant spin
asymmetries at RHIC, which is an important ingredient for the inclusion of RHIC
data in a global analysis of nucleon helicity structure. We confirm the
validity of our calculation by comparing with an existing code that treats the
next-to-leading order cross sections entirely numerically by Monte-Carlo
integration techniques. We also provide new comparisons of the present RHIC
data with results for some of the sets of polarized parton distributions
available in the literature.
