Precision Electroweak Physics (original) (raw)

Precision electroweak measurements on the Z resonance

Geosciences Department Faculty Publication Series, 2006

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 lept eff = 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. The results are compared to the predictions of the Standard Model. 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 Standard Model 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, m t = 173 +13 −10 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 Standard Model. Using in addition the direct measurements of m t and m W , the mass of the as yet unobserved Standard Model Higgs boson is predicted with a relative uncertainty of about 50% and found to be less than 285 GeV at 95% confidence level.

Analysis of electroweak precision data and prospects for future improvements

1998

We update our previous work on an analysis of the electroweak data by including new and partly preliminary data available up to the 1996 summer conferences. The new results on the Z partial decay widths into b and c hadrons now offer a consistent interpretation of all data in the minimal standard model. The value extracted for the strong interaction coupling constant α s (m Z ) agrees well with determinations in other areas. New constraints on the universal parameters S, T and U are obtained from the updated measurements. No signal of new physics is found in the S, T , U analysis once the SM contributions with m t ∼ 175GeV and those of not a too heavy Higgs boson are accounted for. The naive QCD-like technicolor model is now ruled out at the 99%CL even for the minimal model with SU(2) TC . In the absence of a significant new physics effect in the electroweak observables, constraints on masses of the top quark, m t , and Higgs boson, m H , are derived as a function of α s and the QED effective couplingᾱ(m 2 Z ). The preferred range of m H depends rather strongly on the actual value of m t : m H < 360 GeV for m t = 170 GeV, while m H > 130 GeV for m t = 180 GeV at 95 %CL. Prospects due to forthcoming improved measurements of asymmetries, the mass of the weak boson W m W , m t andᾱ(m 2 Z ) are discussed. Anticipating uncertainties of 0.00020 fors 2 (m 2 Z ), 20 MeV for m W , and 2 GeV for m t , the new physics contributions to the S, T , U parameters will be constrained more severely, and, within the SM, the logarithm of the Higgs mass can be constrained to about ±0.35. The better constraints on S, T , U and on m H within the minimal SM should be accompanied with matching precision inᾱ(m 2 Z ).

The minimal set of electroweak precision parameters

Physical Review D, 2006

We present a simple method for analyzing the impact of precision electroweak data above and below the Z-peak on flavour-conserving heavy new physics. We find that experiments have probed about ten combinations of new physics effects, which to a good approximation can be condensed into the effective oblique parameterŝ S,T ,Û , V, X, W, Y (we prove positivity constraints W, Y ≥ 0) and three combinations of quark couplings (including a distinct parameter for the bottom). We apply our method to generic extra Z ′ vectors.

Confronting electroweak precision measurements with New Physics models

The European Physical Journal C, 2000

Precision experiments, such as those performed at LEP and SLC, offer us an excellent opportunity to constrain extended gauge model parameters. To this end, it is often assumed, that in order to obtain more reliable estimates, one should include the sizable one-loop Standard Model (SM) corrections, which modify the Z 0 couplings as well as other observables. This conviction is based on the belief that the higher order contributions from the "extension sector" will be numerically small. However, the structure of higher order corrections can be quite different when comparing the SM with its extension, thus one should avoid assumptions which do not care about such facts. This is the case for all models with ρtree ≡ M 2 W /(M 2 Z cos 2 ΘW ) = 1. As an example, both the manifest left-right symmetric model and the SU (2)L ⊗ U (1)Y ⊗Ũ (1) model, with an additional Z ′ boson, are discussed and special attention to the top contribution to ∆ρ is given. We conclude that the only sensible way to confront a model with the experimental data is to renormalize it self-consistently, if not, parameters which depend strongly on quantum effects should be left free in fits, though essential physics is lost in this way. We should note that arguments given here allow us to state that at the level of loop corrections (indirect effects) there is nothing like a "model independent global analysis" of the data.

Latest results on precision Z physics

Nuclear Physics B - Proceedings Supplements, 1996

The most relevant LEP electroweak measurements obtained during the LEP-I phase are briefly reviewed and their consistency with the Minimal Standard Model (MSM) is discussed. It is shown that only the measurement of the Z partial width into b quarks shows a signifieative discrepancy with respect to its MSM expectation. Assuming the MSM, the precision LEP data alone predict rrtt = (172 +-~0 +1s~_20] GeV, in good agreement with the direct observation at Tevatron. The inclusion of the direct mt measurement in the analysis is shown to open the window for the inference of the Higgs mass from precision data.

Update of electroweak parameters from Z decays

European Physical Journal C, 1993

Based on 520 000 fermion pairs accumulated during the first three years of data collection by the ALEPH detector at LEP, updated values of the resonance parameters of theZ are determined to beM Z =(91.187±0.009) GeV, Γ Z =(2.501±0.012) GeV, σ had 0 =(41.60±0.27) nb, andR ℓ=20.78±0.13. The corresponding number of light neutrino species isN ν=2.97±0.05. The forward-backward asymmetry in lepton-pair decays is used to determine the ratio of vector to axial-vector couplings of leptons:g V 2 (M Z 2 )/g A 2 (M Z 2 )=0.0052±0.0016. Combining this with ALEPH measurements of theb andc quark asymmetries and τ polarization gives sin2θ W eff =0.2326±0.0013. Assuming the minimal Standard Model, and including measurements ofM W /M Z fromp \(\bar p\) colliders and neutrino-nucleon scattering, the mass of the top quark is \(M_{top} = 156 \pm \begin{array}{*{20}c} {22} \\ {25} \\ \end{array} \pm \begin{array}{*{20}c} {17} \\ {22Higgs} \\ \end{array} \) GeV.

Precision Electroweak Physics (original) (raw)

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