Abstract
A measurement of the forward–backward asymmetries of e+e−→bb̄ and e+e−→cc̄ events using electrons and muons produced in semileptonic decays of bottom and charm hadrons is presented. The outputs of two neural networks designed to identify b→ℓ− and c→ℓ+ decays are used in a maximum likelihood fit to a sample of events containing one or two identified leptons. The b and c quark forward–backward asymmetries at three centre-of-mass energies s and the average B mixing parameter χ̄ are determined simultaneously in the fit. Using all data collected by OPAL near the Z resonance, the asymmetries are measured to be: AFBbb̄=(4.7±1.8±0.1)%,AFBcc̄=(−6.8±2.5±0.9)%at〈s〉=89.51 GeV,AFBbb̄=(9.72±0.42±0.15)%,AFBcc̄=(5.68±0.54±0.39)%at〈s〉=91.25 GeV,AFBbb̄=(10.3±1.5±0.2)%,AFBcc̄=(14.6±2.0±0.8)%at〈s〉=92.95 GeV. For the average B mixing parameter, a value of: χ̄=(13.12±0.49±0.42)% is obtained. In each case the first uncertainty is statistical and the second systematic. These results are combined with other OPAL measurements of the b and c forward–backward asymmetries, and used to derive a value for the effective electroweak mixing angle for leptons sin2θeffℓ of 0.23238±0.00052.
Highlights
The measurement of the forward-backward asymmetries of heavy quarks, AqFqB (q=b,c), in e+e− → qq events provides an important test of the Standard Model
The fractions fbqaqck of each quark flavour contributing to the background are taken from the Monte Carlo prediction, as are the dilution factors cdqqilute which take into account the fraction of the primary quark asymmetry that is seen in background events of this flavour
Data from all years were fitted simultaneously, with the various fractions and (NETb, NETc) distributions determined from an appropriate mix of Monte Carlo events with different simulated detector configurations, taking into account the changes in the performance of the OPAL detector over time
Summary
The measurement of the forward-backward asymmetries of heavy quarks, AqFqB (q=b,c), in e+e− → qq events provides an important test of the Standard Model. The bb forward-backward asymmetry provides one of the most precise determinations of the effective electroweak mixing angle for leptons sin θelff (assuming lepton universality). The values of sin θefff are all close to 0.25, so the value of the asymmetry parameter for electrons, Ae, is small, and varies rapidly with sin θelff, but the value of Ab is large, approximately 0.94, and varies only slowly with sin θebff This results in a relatively large forward-backward asymmetry for bb events, which is very sensitive to sin θelff via Ae. This results in a relatively large forward-backward asymmetry for bb events, which is very sensitive to sin θelff via Ae This analysis uses hadronic Z decays observed by the OPAL detector at LEP to measure AbFbB and AcFcB. The charm asymmetry has been measured using reconstructed charm hadrons [11, 12, 13]
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