Improved Direct Measurement of the Parity-Violation Parameter Ab Using a Mass Tag and Momentum-Weighted Track Charge

We present an improved direct measurement of the parity-violation parameter Ab in the Z boson–b quark coupling using a self-calibrating track-charge technique applied to a sample enriched in Z → bb̄ events via the topological reconstruction of the B hadron mass. Manipulation of the SLC electron-beam polarization permits the measurement of Ab to be made independently of other Z-pole coupling parameters. From the 1996-98 sample of 400,000 hadronic Z decays, produced with an average beam polarization of 73.4%, we find Ab = 0.906± 0.022(stat.)± 0.023(syst.). Submitted to Physical Review Letters ∗ Work supportred in part by Department of Energy contract DE-AC03-76SF00515