Electroweak Measurements Using Heavy Quarks Identified in $e^{+}e^{-}$ Annihilation

Since 1989, the Large Electron Positron collider at CERN has been used to study elec- troweak physics to an unprecedented precision. The data have acted as spectacular confirmation of the Standard Model as the best description of electroweak interac- tions at scales of - 100 GeV. However, in 1995, a possible anomaly appeared in the LEP measurement of Rb = (Z-adbb) which was more than three standard devia- ( Z-*hadrons) tions above the Standard Model prediction. This effect could not be accounted for by minor adjustment of model parameters, in particular the mass of the top quark which had recently been directly measured at the Fermilab Tevatron. In order to investigate whether the deviation could be an indication of physics beyond the Standard Model we present new precise measurements of both Rb and the forward-backward asymme- try of b quark production, AbFB , using -63 pb - 1 of data at the Z peak recorded by the L3 detector during 1994-95. The results are: Rb = 0.2146 ± 0.0017(stat) + 0.00 3 3 (sys) - 0.139 (R, - 0.171) AbFB = 9.33 ± 1.40(stat) ± 0.65(sys) ± 0.10(QCD)% This value for Rb agrees with the Standard Model to within one standard deviation. AFB leads to a value for the effective weak mixing angle for b-quarks sin 2 eff - 0.2333 ± 0.0025(stat) ± 0.0012(sys) which is consistent with values obtained using different decay modes of the Z and from neutrino physics, supporting flavour universality. We thus observe no deviation from the Standard Model and, from the Rb measurement, limit the effects of new physics to < 1.7% in b decays