Test Of Lepton Universality Research Articles

A measurement of the [formula omitted] branching ratio

The ρ − → μ − ν μν τ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B( τ − → μ − τ μτ τ) = 0.1734 ± 0.0009(stat) ± 0.0005(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios g μ / g e = 1.0005±0.0043 and g τ / g e = 1.0031 ± 0.0047, in good agreement with the standard Model prediction of unity, and also to determine a value for the Michel parameter η = 0.004± 0.036. This is subsequently used to find a model-dependent limit of the mass for the charged Higgs boson, m H ± > 1.30 tan β, in the Minimal Supersymmetric Standard Model framework.

A measurement of the [formula omitted] branching ratio

The τ−→μ−ν̄μντ branching ratio has been measured using data collected from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value of B(τ−→μ−ν̄μντ)=0.1734±0.0009(stat)±0.0006(syst) has been used in conjunction with other OPAL measurements to test lepton universality, yielding the coupling constant ratios gμ/ge=1.0005±0.0044 and gτ/ge=1.0031±0.0048, in good agreement with the Standard Model prediction of unity. A value for the Michel parameter η=0.004±0.037 has also been determined and used to find a limit for the mass of the charged Higgs boson, mH±>1.28tanβ, in the Minimal Supersymmetric Standard Model.

Tau2000 conference summary

This is a review of some of the highlights of the 6th workshop on the physics of the τ lepton and its neutrino. This includes the test of lepton universality, measurement of Lorentz Structure, study of hadronic decays, direct evidence of tau neutrino, status of neutrino oscillations, and search for neutrinoless decays. This review concludes with a look at the prospect for τ physics in the future.

W properties and decays at LEP2

Preliminary results from W-pair events measured by the four LEP experiments are presented. These results are based on data taken during years 1996–2000 at e +e − centre-of-mass energies between 161–208 GeV and corresponding to a total integrated luminosity of 560 pb −1. A total number of about 7500 W-pair candidates per experiment is used to determine the W decay branching fractions, which are used to test lepton universality to a 1.6% accuracy level. Hadronic decays of W into charm are identified and the fraction of charm decay is 0.494±0.044, corresponding to the CKM matrix element value | V cs|=0.96±0.08. The W mass is measured to be 80.427±0.046 GeV, in agreement with other determinations of this value. The measured value of the W width is Γ W=2.12±0.11 GeV. All results agree with the Standard Model expectations.

Experimental tests of lepton universality in tau decay from CLEO

The branching fractions for τ→eνντ, μνντ, and hντ are measured using data collected with the CLEO detector at the CESR e+e− collider: Be= 0.1776±0.0006±0.0017, Bμ=0.1737±0.0008±0.0018, and Bh=0.1152±0.0005±0.0012, where the first errors are statistical, the second systematic, and h refers either to a charged π or K. Also measured is the tau mass, mτ=(1778.2±1.4) MeV. Lepton universality is affirmed by the resulting relative branching fractions (Bμ/Be=0.9777±0.0063±0.0087, Bh/Be=0.6484±0.0041±0.0060) and the charged-current gauge coupling-constant ratios (gμ/ge=1.0026±0.0055, gτ/gμ=0.9990±0.0098). The tau mass result may be recast as a tau neutrino mass limit, mντ,

Improved measurements of the neutral current from hadron and lepton production at LEP

We present an update with increased statistics to our published analysis of hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine + e − collisions. The published results were based on a total 454 000 hadronic and 58 000 leptonic events. This analysis adds 733 000 hadronic and 88 000 leptonic events recorded at theZ 0 peak in 1992 by the OPAL experiment at LEP. A model independent analysis ofZ 0 parameters based on an extension of the improved Born approximation leads to tests of lepton universality and gives an interpretation of the results within the Standard Model framework. We also present a model independent test for new physics.

Electroweak tests with τ leptons at LEP

We report on two sensitive tests of lepton universality carried out by the 4 LEP experiments at the Z0 pole. From measurements of the τ polarization in e+ e−→τ+τ−, the ratios of the vector and axial vector coupling constants of the electron and the tau lepton to the weak neutral current are obtained to begve/gae=0.066±0.015 andgVτ/gAτ=0.070±0.009 respectively. From measurement of the τ lifetime and the τ leptonic branching ratios, the ratio of the coupling constants describing weak leptonic decays of the τ and the μ is measured to beGτ/Gμ=0.996±0.008.

Precision measurements of the neutral current from hadron and lepton production at LEP

New measurements of the hadronic and leptonic cross sections and of the leptonic forward-backward asymmetries ine+e− collisions are presented. The analysis includes data recorded up to the end of 1991 by the OPAL experiment at LEP, with centre-of-mass energies within ±3 GeV of the Z0 mass. The results are based on a recorded total of 454 000 hadronic and 58 000 leptonic events. A model independent analysis of Z0 parameters based on an extension of the improved Born approximation is presented leading to test of lepton universality and an interpretation of the results within the Standard Model framework. The determination of the mass and width of the Z0 benefit from an improved understanding of the LEP energy calibration.