A measurement of the forward-backward asymmetry of e+e−→bb by applying a jet charge algorithm to lifetime tagged events

The forward-backward asymmetry of$$e^ + e^ - \to Z^0 \to b\bar b$$ has been measured using approximately 2.15 million hadronicZ0 decays collected at the LEP e+e− collider with the OPAL detector. A lifetime tag technique was used to select an enriched$$b\bar b$$ event sample. The measurement of the$$b\bar b$$ asymmetry was then performed using a jet charge algorithm to determine the direction of the primary quark. Values of:$$\begin{gathered} A_{FB}^b = 0.062 \pm 0.034 \pm 0.002 - 0.082\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 89.52GeV, \hfill \\ A_{FB}^b = 0.0963 \pm 0.0067 \pm 0.0038 - 0.471\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 91.25GeV, \hfill \\ A_{FB}^b = 0.172 \pm 0.028 \pm 0.007 - 0.055\Delta (\Gamma _{b\bar b} /\Gamma _{had} ) \hfill \\ at\sqrt s = 92.94GeV, \hfill \\ \end{gathered}$$ were measured where, in each case, the first error is statistical, the second is systematic and the third term gives the variation due to a change$$\Delta (\Gamma _{b\bar b} /\Gamma _{had} )$$ in the value of$$\Gamma _{b\bar b} /\Gamma _{had} = 0.216$$ assumed. The dependence on the assumed charm asymmetry at the same energy is Δ(AFBb)≈+0.07Δ(AFBc). Assuming the Standard Model form for the couplings, these measurements correspond to an effective weak mixing angle of:$$\sin ^2 \theta _W^{eff,e} = 0.2313 \pm 0.0012 \pm 0.0006$$ giving Mtop=196−38−19+33+16GeV/c2, where the first error is statistical and the second is systematic. The Higgs mass assumed is 300 GeV/c2. A variation in the assumed mass of the Higgs boson between 60 and 1000 GeV/c2 corresponds to an uncertainty in sin2θWeff,e of ±0.00006 and on Mtop of−26+20GeV/c2.