A measurement of the parity violating parameter Ab with a muon tag at the SLD

We present a direct measurement of the parity violation parameter A_b, derived from the left-right forward-backward asymmetry of b quarks tagged via unions from semileptonic decays. The value of A_b is extracted using a maximum likelihood fit to the differential cross section for fermion production. The novelty of this measurement consists in the use of topological vertexing information alongside the more traditional decay kinematics to discriminate among the different sources of tagged leptons. The small and stable SLC beam spot and the CCD based vertex detector are used to reconstruct secondary decay vertices and to provide precise kinematic information and a highly efficient and pure B mass tag. A multivariate approach has been used, with a total of 4 tagging variables, whose correlation with each other has been taken into account. The final result has been cross-checked both with a classical cut-and-count method and combining all the information into a neural net. Based on the full SLD dataset of 550K Z"0 events with highly polarized electron beams, this measurement represents an improvement of a factor of 2 with respect to the previously published result (1993-1995 only and with no vertexing information). The statistical sensitivity achieved is around 4% for A_b, making this a world-class single measurement. An estimate of A_c has been simultaneously derived from a common fit, with a precision of about 10%. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:D213678 / BLDSC - British Library Document Supply CentreGBUnited Kingdo