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Abstract
This thesis describes a measurement of the effective weak mixing angle, sin$\sp2\bar\theta\sb{W}$, determined from the forward-backward asymmetry in Z $\to$ bb events collected by the L3 experiment at LEP during 1990 and 1991. The data sample corresponds to approximately 410,000 hadronic decays of the Z.
The b quarks were identified by their semileptonic decay to electrons or muons. The transverse momentum of the lepton with respect to the axis of the nearest jet, $p\sb{T}$, was used to distinguish between leptons from b decay and those from background.
Due to the mixing of neutral B mesons, the observed asymmetry is reduced by a factor (1-2$\chi$), where $\chi$ is the B$\sp0$-B$\sp0$ mixing parameter. A determination of $\chi$ from the data sample in which both b quarks in the event were tagged by a lepton is also presented. A fit to the dilepton $p\sb{T}$ spectra resulted in $\chi$ = 0.125 $\pm$ 0.021(stat) $\pm$ 0.012(syst).
From a fit to the single lepton $p\sb{T}$ spectrum the bb forward-backward asymmetry was determined to be $A\sb{\rm b\bar b}$ = 0.085 $\pm$ 0.017(stat) $\pm$ 0.004(syst), corrected for B$\sp0$-B$\sp0$ mixing. This value corresponds to a mixing angle sin$\sp2\bar\theta\sb{W}$ = 0.2336 $\pm$ 0.0032, which is in excellent agreement with independent measurements of the mixing angle measured from other L3 data.
