A Determination of the Effective Weak Mixing Angle From a Measurement of the Forward-Backward Charge Asymmetry in Heavy Flavour Decays of the Z0 Boson at LEP

[摘要] This thesis describes a measurement of the forward-backward hemisphere charge asymmetry in heavy flavour decays of the Z0 boson using the ALEPH detector at LEP. Z0→ bb decays are tagged by identifying the decay products of long-lived b hadrons via. their large track impact parameters relative to the Z0 decay point. The background event tagging efficiencies are estimated using Monte Carlo simulation, whereas the bb event tagging efficiency is evaluated by measuring the relative rates of single and double tagged events. In a sample of 70,259 hadronic Z0 decays, with a b purity of 88%, the charge asymmetry, , expressed as the mean difference between the momentum weighted forward and backward hemisphere charges, is measured to be= -0.0112 +/- 0.00 12(stat.) +/-0.00018(syst.). Using a precision measurement of the mean b quark hemisphere charge, and accurate fits to the angular dependence of the event tagging efficiencies,is used to determine a value for the electroweak asymmetry,: AbbFB= 0.1183+/- 0.0131(stat.) +/- 0.00057(syst.). Interpreting AbbFB in the framework of the minimal Standard Model of electroweak interactions, the effective weak mixing angle. Sin2thetaWeff is determined : Sin2thetaWeff = 0.2281 +/- 0.0024(stat) +/- 0.0011 (syst). This places the following Standard Model limits on the mass of the top quark, mt = 259+43-50GeV/c2, or mt < 338 GeV/c2, (95% confidence level).

[发布机构] University:University of Glasgow

[关键词] High energy physics