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, mZmZ and ΓZΓZ, and its couplings to fermions, for example the ρρ parameter and the effective electroweak mixing angle for leptons, are precisely measured:mZ=91.1875±0.0021GeV,ΓZ=2.4952±0.0023GeV,ρℓ=1.0050±0.0010,sin2θefflept=0.23153±0.00016.The number of light neutrino species is determined to be 2.9840±0.00822.9840±0.0082, in agreement with the three observed generations of fundamental fermions.The results 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.Through 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, mt=173-10+13GeV, and the mass of the W boson, mW=80.363±0.032GeV. These indirect constraints are compared to the direct measurements, providing a stringent test of the SM. Using in addition the direct measurements of mtmt and mWmW, 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 285GeV at 95% confidence level.