Measurement of the hadronic cross section at the Z resonance and determination of electroweak parameters

• Main Author: You, Jianming
• Format: Dissertation
• Language: English
• Subjects: physics, elementary particles and high energy
• ISBN: 9780591082265; 0591082268

We determine the hadronic cross section at $\sqrt{s}=91.224$ GeV:$$\rm\sigma\sb{had}=30.522\pm0.027\ (stat.)\pm0.014\ (sys.)\pm0.06\ (lumi.)\ nb,$$using L3 data taken in 1994. We extract the electroweak parameters from the leptonic and hadronic cross sections and the leptonic forward-backward asymmetries at the Z peak. All our measurements support the hypothesis of lepton universality. From the cross sections, we determine the properties of the Z boson:$$\eqalign{m\rm\sb{Z}=91192.1\pm3.6\ &\rm MeV,\quad\Gamma\sb{Z}=2501\pm5\ MeV,\cr \rm\Gamma\sb{had}=1750\pm5\ &\rm MeV,\quad \Gamma\sb{\ell}=83.13\pm0.23\ MeV.}$$The corresponding invisible width of 499.1 $\pm$ 4.0 MeV constrains the number of light neutrino species to be: $N\sb{\nu}=2.990 \pm 0.020$ within the Standard Model. Including the leptonic forward-backward asymmetries, we find:$${\rm\sigma\sbsp{had}{o}=41.54\pm0.10\ nb},\quad R\sbsp{\rm had}{\ell}=20.80\pm0.08,\quad A\sbsp{\rm fb}{\rm o,\ell}=0.0182\pm0.0029.$$Including the average tau polarization ${\cal P}\sb{\tau}$ measurement, the effective neutral weak current coupling constants for charged leptons are found to be:$$\bar g\rm\sbsp{V}{\ell}={-}0.0389\pm0.0023,\quad\bar g\sbsp{\rm A}{\ell}={-}0.5016\pm0.0007.$$Using the S-matrix ansatz, we determine the $\gamma$-Z interference term $j\rm\sbsp{had}{tot}=0.05\pm0.39.$ All our measurements are in good agreement with the predictions of the Standard Model.
Within the Standard Model, we determine:$$\eqalign{\alpha\sb{\rm s}(m\sb{\rm Z})&=0.121\pm0.008\pm0.002\ ({\rm Higgs}),\cr m\sb{\rm t}&=203\pm19\pm17\ ({\rm Higgs})\ {\rm GeV}.}$$Subsequently, we derive measurements of the effective electroweak mixing angle and the W boson mass:$$\sin\sp2\bar\theta\sb{\rm W}=0.2310\pm0.0007,\quad m\sb{\rm W}=80.51\pm0.15\ {\rm GeV}.$$.