Precision electroweak measurements on the Z resonance

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, m Z and Γ Z , and its couplings to fermions, for example the ρ parameter and the effective electroweak mixing angle for leptons, are precisely measured: m Z = 91.1875 ± 0.0021 GeV , Γ Z = 2.4952 ± 0.0023 GeV , ρ ℓ = 1.0050 ± 0.0010 , sin 2 θ eff lept = 0.23153 ± 0.00016 . The number of light neutrino species is determined to be 2.9840 ± 0.0082 , in agreement with the three observed generations of fundamental fermions. sults are compared to the predictions of the Standard Model (SM). At the Z-pole, 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 SM expectation, at the level of 2.8 standard deviations. h radiative corrections evaluated in the framework of the Standard Model, the Z-pole data are also used to predict the mass of the top quark, m t = 173 - 10 + 13 GeV , and the mass of the W boson, m W = 80.363 ± 0.032 GeV . These indirect constraints are compared to the direct measurements, providing a stringent test of the SM. Using in addition the direct measurements of m t and m W , the mass of the as yet unobserved SM Higgs boson is predicted with a relative uncertainty of about 50% and found to be less than 285 GeV at 95% confidence level.