The forward-backward asymmetry of$e^ + e^ - \to b\bar b and e^ + e^ - \to c\bar c$ using leptons in hadronicZ ⁰ decays

Abstract

The forward-backward asymmetries of$e^ + e^ - \to Z^0 \to b\bar b and e^ + e^ - \to Z^0 \to c\bar c$ have been measured by the OPAL Collaboration using samples of hadronicZ ⁰ decays in which electron or muon candidates were observed. The asymmetries were measured simultaneously in a two parameter fit, which used the distributions of the track momentum and transverse momentum component with respect to the associated jet to distinguish lepton candidates from different sources. From a sample of 360 000 hadronic events with centre-of-mass energies within ±0.5 GeV of theZ ⁰ mass and mean energy 91.24. GeV, the values obtained for the$b\bar b$ asymmetry before and after correcting, for the effect of$B^0 \overline {B^0 }$ mixing, and for the$c\bar c$ asymmetry are, respectively:

$$\begin{gathered} A_{FB}^{b,mix} = 0.070 \pm 0.014(stat) \pm 0.005(sys), \hfill \\ A_{FB}^b = 0.092 \pm 0.018(stat) \pm 0.007(sys) \hfill \\ \pm 0.003(mix), \hfill \\ A_{FB}^c = 0.014 \pm 0.030(stat) \pm 0.020(sys). \hfill \\ \end{gathered} $$

The measurement of the$b\bar b$ asymmetry is confirmed by a measurement using only leptons with high transverse momentum. This sample is highly enriched in semileptonic decays ofb hadrons. Measurements using high transverse momentum leptons were also performed using samples of events with centre-of-mass energies further from theZ ⁰ mass. After correcting for the effect of$B^0 \overline {B^0 }$ mixing, the results are:

$$\begin{gathered} A_{FB}^b (\langle \sqrt s \rangle = 89.66GeV) \hfill \\ = 0.071 \pm 0.054(stat) \pm 0.007(sys) \pm 0.002(mix), \hfill \\ A_{FB}^b (\langle \sqrt s \rangle = 92.75GeV) \hfill \\ = 0.131 \pm 0.047(stat) \pm 0.012(sys) \pm 0.004(mix). \hfill \\ \end{gathered} $$