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

TL;DR

This work delivers a comprehensive LEP-wide combination of high-energy electroweak measurements from LEP-II in the range $130$ to $209~\mathrm{GeV}$, including photon-pair and fermion-pair production, four-fermion processes, and W/Z self-couplings, together with a preliminary W-boson mass and width combination. It employs BLUE to average cross-sections and asymmetries, integrates correlations across experiments, and uses ZFITTER for Standard Model predictions to interpret results and derive constraints. Across the photon-pair and fermion-pair channels, no significant deviations from the SM are observed; the analysis yields 95% CL limits on various new-physics scenarios (e.g., contact interactions, excited electrons, large extra dimensions) and tightens the W-mass constraints when combined with Z-pole data. These results refine tests of the Standard Model and provide predictions for low-$Q^2$ observables, reinforcing the consistency of the electroweak sector at high energies.

Abstract

This note presents a combination of published and preliminary electroweak results from the four LEP collaborations ALEPH, DELPHI, L3 and OPAL based on electron-positron collision data taken at centre-of-mass energies above the Z-pole, $130 \GeV$ to $209 \GeV$ (\LEPII), as prepared for the 2005 summer conferences. Averages are derived for di-fermion cross sections and forward-backward asymmetries, photon-pair, W-pair, Z-pair, single-W and single-Z cross sections, electroweak gauge boson couplings, W mass and width and W decay branching ratios. An investigation of the interference of photon and Z-boson exchange is presented, and colour reconnection and Bose-Einstein correlation analyses in W-pair production are combined. The main changes with respect to the experimental results presented in 2004 are updates to some 4-fermion cross sections, final results on BE correlations and a new preliminary combination of the mass and width of the W boson. Including the precision electroweak measurements performed at the Z pole published recently, the results are compared with precise electroweak measurements from other experiments, notably CDF and DØat the Tevatron. Constraints on the input parameters of the Standard Model are derived from the results obtained in high-$Q^2$ interactions, and used to predict results in low-$Q^2$ experiments, such as atomic parity violation, Moller scattering, and neutrino-nucleon scattering.

Figures (7)

• Figure 1: Cross-section ratios $r = \sigma_{\rm meas} / \sigma_{\rm QED}$ at different energies. The measurements of the single experiments are displaced by $\pm$ 200 or 400 $\mathrm{Me V}$ from the actual energy for clarity. Filled symbols indicate published results, open symbols stand for preliminary numbers. The average over the experiments at each energy is shown as a star. Measurements between 203 and 209 $\mathrm{Ge V}$ are averaged to one energy point. The theoretical error is not included in the experimental errors but is represented as the shaded band.
• Figure 2: Examples for angular distributions of the four LEP experiments. Points are the data and the curves are the QED prediction (solid) and the individual fit results for $\Lambda_{\pm}$ (dashed). ALEPH shows the uncorrected number of observed events, the expectation is presented as histogram.
• Figure 3: 95% CL limits on the coupling $f_\gamma/\Lambda$ of an excited electron as a function of $M_{\rm e^\ast}$. In the case of $f=f'$ it follows that $|f_\gamma| = f$. It is assumed that $\Lambda=M_{\rm e^\ast}$.
• Figure 5: Diagrams leading to the the production of initial state non-singlet electron-positron pairs in $\mathrm{e}^+\mathrm{e}^-\rightarrow\mu^+\mu^-$, which are considered as signal in the common signal definition.
• Figure 6: Preliminary combined LEP results on the cross-sections for $\mathrm{q\overline{q}}$, $\mu^+\mu^-$ and $\tau^+\tau^-$ final states, as a function of centre-of-mass energy. The expectations of the SM, computed with ZFITTER [ff:ref:ZFITTER], are shown as curves. The lower plot shows the ratio of the data divided by the SM.