Precision Electroweak Measurements on the Z Resonance

TL;DR

The study presents the final high-precision electroweak measurements at the Z resonance using LEP and SLC data, enabling stringent tests of the Standard Model. It reports precise determinations of Z parameters (mass, width) and couplings, the number of light neutrino species, and clear evidence for electroweak radiative corrections at high significance. A notable tension appears in the forward-backward asymmetry for b-quarks, while indirect constraints on the top quark and W boson masses are derived and cross-checked against direct measurements; combining these with the Higgs sector yields a bound on the Higgs mass. Overall, the results provide a comprehensive validation of the SM at the loop level and impose tight constraints on possible new physics.

Abstract

We report on the final electroweak measurements performed with data taken at the Z resonance by the experiments operating at the electron-positron colliders SLC and LEP. The data consist of 17 million Z decays accumulated by the ALEPH, DELPHI, L3 and OPAL experiments at LEP, and 600 thousand Z decays by the SLD experiment using a polarised beam at SLC. The measurements include cross-sections, forward-backward asymmetries and polarised asymmetries. The mass and width of the Z boson, $\MZ$ and $\GZ$, and its couplings to fermions, for example the $ρ$ parameter and the effective electroweak mixing angle, are precisely measured. The number of light neutrino species is determined to be 2.9840+/-0.0082. The results are compared to the predictions of the Standard Model. Electroweak radiative corrections beyond the running of the QED and QCD coupling constants are observed with a significance of five standard deviations, and in agreement with the Standard Model. Of the many Z-pole measurements, the forward-backward asymmetry in b-quark production shows the largest difference with respect to its Standard Model expectation, at the level of 2.8 standard deviations. Through radiative corrections evaluated in the framework of the Standard Model, the masses of the top quark and the W Boson are predicted. These indirect constraints are compared to the direct measurements, providing a stringent test of the Standard Model. Using in addition the direct measurements of $\Mt$ and $\MW$, the mass of the as yet unobserved Standard Model Higgs boson is predicted.

Figures (7)

• Figure 1: The lowest-order $s$-channel Feynman diagrams for $\mathrm{e}^+\mathrm{e}^- \rightarrow \mathrm{f\overline{f}}$. For $\mathrm{e}^+\mathrm{e}^-$ final states, the photon and the Z boson can also be exchanged via the $t$-channel. The contribution of Higgs boson exchange diagrams is negligible.
• Figure 2: The hadronic cross-section as a function of centre-of-mass energy. The solid line is the prediction of the $\mathrm{SM}$, and the points are the experimental measurements. Also indicated are the energy ranges of various $\mathrm{e}^+\mathrm{e}^-$ accelerators. The cross-sections have been corrected for the effects of photon radiation.
• Figure 3: The LEP storage ring, showing the locations of the four experiments, and the PS and SPS accelerators used to pre-accelerate the electron and positron bunches.
• Figure 4: The SLC linear collider complex, showing the electron source, the damping rings, the positron source, the 3 km long linac and arcs and the final focus. The helix and arrow superimposed on the upper arc schematically indicate the electron spin precession which occurs during transport.
• Figure 5: The amount of longitudinal electron polarisation as a function of the number of recorded Z decays at SLD.