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

TL;DR

The note presents a comprehensive LEP-II electroweak data combination for photon-pair and fermion-pair production, synthesizing results from ALEPH, DELPHI, L3, and OPAL with BLUE and ZFITTER frameworks. It tests QED and the Standard Model against high-energy e+e− collisions (130–209 GeV) and derives 95% CL limits on a range of new-physics scenarios, including contact interactions, excited electrons, and extra dimensions. No significant deviations from SM predictions are observed, reinforcing the SM and constraining possible beyond-SM effects at the TeV scale. The results, together with Z-pole measurements and Tevatron data, tighten the global electroweak parameter fits and inform expectations for low-Q² processes.

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 2006 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 2005 are new preliminary combinations of final {\LEPII} results on 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, M{ö}ller 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 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.