An inclusive analysis of the leptonic decay modes of the Z⁰ boson

Thesis



This thesis describes an analysis of the process e⁺e^— → l⁺l^- (where l = e, μ,, τ) at centre-of-mass energies between 88 GeV and 94 GeV, using the data collected by the DELPHI detector between the years 1991 and 1993. The leptonic decays of the Z° boson are selected without attempting to separate the three lepton types, thus making it an inclusive lepton analysis.

The theory behind lepton pair production is introduced and the extraction of various electroweak parameters from the experimental observables is discussed. The LEP collider and the DELPHI detector are described, with special emphasis being given to the sub-detectors used in the analysis. The criteria used to select a high purity leptonic sample are described along with calculations of various backgrounds and efficiencies. The sample of selected leptonic events is then used to measure the cross-sections and forward-backward asymmetries.

Finally, a fit to these cross-sections and asymmetries, together with the hadronic (e⁺e^- → qq̄) cross-sections, is carried out. Various Z° parameters are obtained: the mass and total width Mz-91.1876 ± 0.0052 GeV/c², Γ_z = 2.4971 ± 0.0061 GeV, the ratio of the hadronic to leptonic partial widths R_l = 20.73 ± 0.09, and the pole leptonic asymmetry (A°_FB)¹ = 0.0195 ± 0.0042. Using these results and the value of the strong coupling constant (α_s), determined by the DELPHI collaboration, the number of light neutrino species is determined to be N_ν = 3.045 ± 0.035. The leptonic partial width is found to be: Γ_l = 83.82 ± 0.29 MeV. Using the measured leptonic forward-backward asymmetries, the squared vector and axial-vector couplings of the Z° to charged leptons are found to be (ĝ_v¹)²-3

and (ĝ_a¹)2 = 0.2505 ± 0.0009. These values can be used to determine the effective rho parameter and the effective weak mixing angle: p̂ = 1.0020 ± 0.0036, and sin² θ^eff_lept = 0.2297 ± 0.0024.

A full Standard Model fit to the data gives the values of the strong coupling constant, α_s, and the mass of the top quark, m_top, as being:

α_s = 0.123 ± 0.010,

m_top = 178⁺²²_-25 (expt)^{+18_-16(Higg s)GeV/c2,}

where 60 < m_Higgs (GeV/c²) < 1000 with a central value of 300 GeV/c². All the results obtained agree with the results from the lepton-identified analyses (analyses in which leptonic events are selected on the basis of their individual flavour) and with the predictions of the Standard Model.

Authors: Zuberi, R; Rashid Shahid Zuberi

• Publication date: 1994
• DOI: 10.5287/ora-pm9zwonkq
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Particles (Nuclear physics); Experiments; Leptons (Nuclear physics); Decay; Z bosons