Electroweak Effects in $e^+ e^- \to \mu^+ \mu^-$ at 29-{GeV}

A measurement of the cross section for production of collinear muon pairs based upon a sample of about 3000 events observed in the MAC detector at the storage ring PEP is presented. From the angular asymmetry ${A}_{\ensuremath{\mu}\ensuremath{\mu}}=0.076\ifmmode\pm\else\textpm\fi{}0.018$ the axial-vector weak neutral coupling is found to be given by ${{g}_{A}}^{e}{{g}_{A}}^{\ensuremath{\mu}}=0.31\ifmmode\pm\else\textpm\fi{}0.08$.