A Combination of Preliminary Electroweak Measurements and Constraints on the Standard Model

TL;DR

The study presents a comprehensive LEP- and SLD-era electroweak data synthesis, combining Z-pole observables, W properties, heavy-flavour measurements, and high-energy f f̄ production to test the Standard Model and constrain its parameters. Through multi-parameter fits (including S-Matrix and Z′/contact-interaction interpretations) and precise determinations of m_W, m_t, and sin^2 θ_eff^lept, the analysis reveals general SM consistency while favoring a relatively light Higgs and predicting tight limits on new physics. The work highlights the power of cross-experiment combinations to reduce systematics and sharpen indirect constraints on fundamental parameters. It also outlines how current data constrain potential Z′ bosons, anomalous gauge couplings, and quartic interactions, with prospects for further refinement as LEP-II data accumulate.

Abstract

This note presents a combination of published and preliminary electroweak results from the four LEP collaborations and the SLD collaboration which were prepared for the 2000 summer conferences. Averages from Z0 resonance results are derived for hadronic and leptonic cross sections, the leptonic forward-backward asymmetries, the tau polarisation asymmetries, the bbbar and ccbar partial widths and forward-backward asymmetries and the qqbar charge asymmetry. Above the Z0 resonance, averages are derived for di-fermion cross sections and asymmetries, W-pair, Z-pair and single-W production cross section, electroweak gauge boson couplings and W mass and decay branching ratios. The major changes with respect to results presented in summer 1999 are final Z0 lineshape results from LEP, updates to the W mass and gauge-boson couplings from LEP, and ALR from SLD. The results are compared with precise electroweak measurements from other experiments. The parameters of the Standard Model are evaluated, first using the combined LEP electroweak measurements, and then using the full set of electroweak results.

Figures (26)

• Figure 1: Contours of 68% probability in the $R_{\ell}$-$A^{0,\,\ell}_{\rm {FB}}$ plane. For better comparison the results for the $\tau$ lepton are corrected to correspond to the massless case. The $\hbox{Standard Model}$ prediction for $m_{\mathrm{Z}}=91.1875$Ge V, $m_{\mathrm{t}}=174.3$Ge V, $m_{\mathrm{H}}=300$Ge V, and $\alpha_{\rm{s}}(m_{\mathrm{Z}}^2)=0.119$ is also shown. The lines with arrows correspond to the variation of the $\hbox{Standard Model}$ prediction when $m_{\mathrm{t}}$, $m_{\mathrm{H}}$ and $\alpha_{\rm{s}}(m_{\mathrm{Z}}^2)$ are varied in the intervals $m_{\mathrm{t}}=174.3\pm5.1$Ge V, $m_{\mathrm{H}}=300^{+700}_{-187}~\mathrm{ Ge V} \textrm{Ge V}$, and $\alpha_{\rm{s}}(m_{\mathrm{Z}}^2)=0.119\pm0.002$, respectively. The arrows point in the direction of increasing values of $m_{\mathrm{t}}$, $m_{\mathrm{H}}$ and $\alpha_{\rm{s}}$.
• Figure 2: Contours in the $R{\mathrm{b}}^0$R_b^0$$-$R{\mathrm{c}}^0$R_c^0$$ plane derived from the LEP+SLD data, corresponding to 68% and 95% confidence levels assuming Gaussian systematic errors. The Standard Model prediction for $m_{\mathrm{t}}=174.3 \pm 5.1$Ge V is also shown. The arrow points in the direction of increasing values of $m_{\mathrm{t}}$.
• Figure 3: Preliminary combined LEP results on the cross sections for $\mathrm{q\bar{q}}$, $\mu^+\mu^-$ and $\tau^+\tau^-$ final states, and forward-backward asymmetries for $\mu^+\mu^-$ and $\tau^+\tau^-$ final states as a function of centre-of-mass energy. The values at 130--189 GeV are taken from ref:lep2-ff-moriond2000. The prediction of the Standard Model, computed with ZFITTER ref:lep2-ff-zfitter, are shown as curves. The lower plots shows the ratio of the data divided by the predictions for the cross sections, and the difference between the measurements and the predictions for the asymmetries. For better visibility, the $\mu^+\mu^-$ and $\tau^+\tau^-$ data points are slightly shifted apart from their nominal centre-of-mass value.
• Figure 4: LEP averaged differential cross sections for $\mathrm{e}^+\mathrm{e}^- \rightarrow \mu^+\mu^-$ at energies of 183-202 GeV. The Standard Model Predictions shown as solid histograms are computed with ZFITTER ref:lep2-ff-zfitter
• Figure 5: LEP averaged differential cross sections for $\mathrm{e}^+\mathrm{e}^- \rightarrow \tau^+\tau^-$ at energies of 183-202 GeV. The Standard Model Predictions shown as solid histograms are computed with ZFITTER ref:lep2-ff-zfitter