Improved Direct Measurement of $A_b$ and $A_c$ at the $Z^0$ Pole Using a Lepton Tag

The parity violation parameters $A_b$ and $A_c$ of the $Zb{\bar b}$ and $Zc\bar{c}$ couplings have been measured directly, using the polar angle dependence of the polarized cross sections at the $Z^0$ pole. Bottom and charmed hadrons were tagged via their semileptonic decays. Both the electron and muon analyses take advantage of new multivariate techniques to increase the analyzing power. Based on the 1993-98 SLD sample of 550,000 $Z^0$ decays produced with highly polarized electron beams we measure $A_b = 0.919 \pm 0.030_{stat} \pm 0.024_{syst}$, and $A_c = 0.583 \pm 0.055_{stat} \pm 0.055_{syst}$.