Measurements of the tau lepton polarization and forward-backward polarization asymmetry near the Z resonance using the OPAL detector are described. The measurements are based on analyses of tau -> e nu_e nu_tau, tau -> mu nu_mu nu_tau, tau -> pi nu_tau, tau -> rho nu_tau and tau -> a1 nu_tau decays from a sample of 144810 e+e- -> tau+ tau- candidates corresponding to an integrated luminosity of 151 pb-1. Assuming that the tau lepton decays according to V-A theory, we measure the average tau polarization near Ecm = MZ to be <Ptau> = (-14.10 +/- 0.73 +/- 0.55)% and the tau polarization forward-backward asymmetry to be Afb = (-10.55 +/- 0.76 +/- 0.25)%, where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: Atau = 0.1456 +/- 0.0076 +/- 0.0057, Ae = 0.1454 +/- 0.0108 +/- 0.0036. These measurements are consistent with the hypothesis of lepton universality and combine to give Al = 0.1455 +/- 0.0073. Within the context of the Standard Model this combined result corresponds to sin^2(theta)(lept,effective) = 0.23172 +/- 0.00092. Combing these results with those from the other OPAL neutral current measurements yields a value of sin^2(theta)(lept,effective) = 0.23211 +/- 0.00068.
No description provided.
This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in e+e- collisions with the OPAL detector makes use of the full LEP1 data sample comprising 161 pb^-1 of integrated luminosity and 4.5 x 10^6 selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from Z-gamma interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: mZ = 91.1852 +- 0.0030 GeV, GZ = 2.4948 +- 0.0041 GeV, s0h = 41.501 +- 0.055 nb, Rl = 20.823 +- 0.044, and Afb0l = 0.0145 +- 0.0017. Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, Ginv/Gl = 5.942 +- 0.027. Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, N_nu = 2.984 +- 0.013. Interpreting the data within the context of the Standard Model allows the mass of the top quark, mt = 162 +29-16 GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of mt as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: alfa_s(mZ) = 0.127 +- 0.005 and mH = 390 +750-280 GeV.
The cross section for hadron production corrected to the simple kinematic acceptance region defined by SPRIME/S > 0.01. Statistical errors only are shown. Also given is the cross section value corrected for the beam energy spread to correspond to the physical cross section at the central value of SQRT(S).
The cross section for E+ E- production corrected to the simple kinematic acceptance region defined by ABS(COS(THETA(C=E-))) < 0.7 and THETA(C=ACOL) < 10 degrees. Statistical errors only are shown. Also given is the cross section value corrected for the beam energy spread to correspond to the physical cross sectionat the central value of SQRT(S).
The cross section for mu+ mu- production corrected to the simple kinematic acceptance region defined by N = M(P=3_4)**2/S > 0.01. Statistical errors only are shown. Also given is the cross section value corrected for the beam energy spread to correspond to the physical cross section at the central value of SQRT(S).
We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993-95. A total luminosity of 103 pb^-1 was collected at centre-of-mass energies \sqrt{s} ~ m_Z and \sqrt{s} ~ m_Z +/- 1.8 GeV which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990-92, we obtain the final results: m_Z = 91189.8 +/- 3.1 MeV, Gamma_Z = 2502.4 +/- 4.2 MeV, Gamma_had = 1741.1 +/- 3.8 MeV, Gamma_l = 84.14 +/- 0.17 MeV. An invisible width of Gamma_inv = 499.1 +/- 2.9 MeV is derived which in the Standard Model yields for the number of light neutrino species N_nu = 2.978 +/- 0.014. Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be \bar{g}_V^l = -0.0397 +/- 0.0017 and \bar{g}_A^l = -0.50153 +/- 0.00053.Including our measurements of the Z -> b \bar{b} forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of sin^2\bar{\theta}_W = 0.23093 +/- 0.00066 is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of m_H < 133 GeV is set at 95% confidence level.
Updated values of coupling constants and electroweak mixing angle.
Cross sections for hadron production from the 1993 data. The first DSYS error is the uncorrelated part of the systematic error. The second DSYS error is from the statistical error on the absolute luminosity. In addition there is a fully correlated multiplicative contribution to the systematic error of 0.039 PCT plus an absolute uncertainty of 3.2pb together with an additional error from the absolute luminosity of 0.105 PCT.
Cross sections for hadron production from the 1994 data. The first DSYS error is the uncorrelated part of the systematic error. The second DSYS error is from the statistical error on the absolute luminosity. In addition there is a fully correlated multiplicative contribution to the systematic error of 0.039 PCT plus an absolute uncertainty of 3.2pb together with an additional error from the absolute luminosity of 0.088 PCT.
We have measured both the rates and the forward-backward asymmetry of ℓ + ℓ − from Z 0 →ℓ + ℓ − (where ℓ= μ , τ ) with the L3 detector. We obtained Γ ℓℓ =88±4±3 MeV and the vector neutral current coupling constant, g v =0.00±0.07 and the axial vector neutral current coupling constant, g A =−0.515±0.015.
No description provided.
No description provided.
We have measured the properties of Z 0 → b b decays using a sample of 944 inclusive muon events, corresponding to 18 000 hadron events obtained with the L3 detector at LEP. We measured the partial decay width of the Z 0 into b b , Γ b b =353±48 MeV , and we determined the vector coupling of the Z 0 to the b quark; g rmv 2 (b)=0.095±0.047. We measured the forward-backward charge asymmetry in e + e − → b b events at √ s ≈ M v , and obtained A b b =13.3±9.9% .
BOTTOM quark charge asymmetry measurement.
The search for an additional heavy gauge boson Z′ is described. The models considered are based on either a superstring-motivated E 6 or on a left-right symmetry and assume a minimal Higgs sector. Cross sections and asymmetries measured with the L3 detector in the vicinity of the Z resonance during the 1990 and 1991 running periods are used to determine limits on the Z-Z′ gauge boson mixing angle and on the Z′ mass. For Z′ masses above the direct limits, we obtain the following allowed ranges of the mixing angle, θ M at the 95% confidence level: −0.004 ⪕ θ M ⪕ 0.015 for the χ model, −0.003 ⪕ θ M ⪕ 0.020 for the ψ model, −0.029 ⪕ θ M ⪕ 0.010 for the η model, −0.002 ⪕ θ M ⪕ 0.020 for the LR model,
Data taken during 1990.
Data taken during 1991.
Data taken during 1990.
We present a measurement of the forward-backward charge asymmetry in hadronic decays of the Z 0 using data collected with the OPAL detector at LEP. The forward-backward charge asymmetry was measured using a weight function method which gave the number of forward events on a statistical basis. In a data sample of 448 942 hadronic Z 0 decays, we have observed a charge asymmetry of A h = 0.040±0.004 (stat.)±0.006 (syst.)±0.002 (B 0 B 0 mix.), taking into account the effect of B 0 B 0 mixing. In the framework of the standard model, this asymmetry corresponds to an effective weak mixing angle averaged over five quark flavours of sin 2 θ W = 0.2321 ± 0.0017 ( stat. ) ± 0.0027 ( syst. ) ± 0.0009 (B 0 B 0 mix.). The result agrees with the value obtained from the Z 0 line shape and lepton pair forward-backward asymmetry.
No description provided.
The second systematic error is due to the uncertainty in the correction for B.BBAR mixing which had been applied to the data.
The second systematic error is due to the uncertainty in the correction for B.BBAR mixing which had been applied to the data.
From a sample of 150 000 hadronic Z decays collected with the ALEPH detector at LEP, events containing prompt leptons are used to measure the forward-backward asymmetries for the channels Z → b b and Z → c c , giving the results A FB b =0.126±0.028±0.012 and A FB c =0.064±0.039±0.030. These asymmetries correspond to the value of effective electroweak mixing angle at the Z mass sin 2 θ W ( m Z 2 ) = 0.2262±0.0053.
b asymmetry from high pt leptons.
b asymmetry from full pt range.
b asymmetry from full pt range.
A significant charge asymmetry is observed in the hadronic Z decays with the ALEPH detector at LEP. The asymmetry expressed in terms of the difference in momentum weighted charges in the two event hemispheres is measured to be < Q forward >−< Q backward >= −0.0084±0.0015 (stat.) ±0.0004 (exp. sys.). In the framework of the standard model this can be interpreted as a measurement of the effective electroweak mixing angle, sin 2 O w ( M z 2 =0.2300±0.0034 (stat.) ±0.0010 (exp. sys.) ±0.0038 (theor. sys.) or of the ratio of the vector to axual- vector coupling costants of the electron, g ve g Ae =+0.073±0.024.
No description provided.
No description provided.
We have measured the forward-backward asymmetry in Z 0 → b b decays using hadronic events containing muons and electrons. The data sample corresponds to 118 200 hadronic events at √ s ≈ M z . From a fit to the single and dilepton p and P ⊥ spectra, we determine A b b =0.130 −0.042 +0.044 including the correction for B 0 − B 0 mixing.
Observed asymmetry from fit to single and dilepton P and PT spectra assuming no mixing.
Asymmetry corrected for the effects of mixing using the L3 observed mixing parameter chi(B) = 0.178 +0.049,-0.040.
SIN2TW determined from the asymmetry measurement.
Forum