A Study of the Forward-Backward Charge Asymmetry in Hadronic Z0 Decays

[摘要] This thesis concerns a study of the forward-backward charge asymmetry observed in hadronic events using the ALEPH detector at the LEP e+e- collider. The asymmetry is interpreted as arising from electroweak effects during quark production in Z0 decays. Particle charges and momenta are measured using information from the time projection and inner tracking chambers of ALEPH while the effects of charge retention are investigated and used to tag the charge and direction of quarks from particles detected after parton fragmentation. The data were recorded over a range of energies around the Z0 resonance and represent an integrated luminosity of 8.7+/-0.2 pb-1. The asymmetry is expressed as the mean difference between momentum weighted charges in the forward and backward hemispheres and is found to be : (Forward Charge --- Backward Charge) = -0.0084+/-0.0014 (statistical)+/-0.0004 (expt. syst.) Fragmentation models are used to estimate the degree of charge retention for each quark flavour so that their contributions to the observed asymmetry can be determined. In the context of the Standard Model, the asymmetry may be interpreted in terms of the coupling strength of fermions to the Z0 and so as a measure of the effective electroweak mixing angle: sin2thetaw(M2z)) = 0.2303+/-0.0036(stat.) +/-0.0009 (expt. syst.) +/-0.0038 (theor. syst.) When taken in conjunction with previously extracted quark coupling constants from neutrino-nucleon scattering measurements, the ratio of lepton couplings is found to be : Ve/ae= +0.072+/-0.025 combining both statistical and systematic uncertainties. This measurement establishes that the axial and vector lepton couplings have the same sign.

[发布机构] University:University of Glasgow

[关键词] High energy physics