The Mark J Collaboration at the DESY e+e− collider PETRA presents results on the electroweak reactions e+e−→μ+μ−τ+τ−,μ+μ−γ, and e+e−μ+μ−. The c.m. energy range is 12 to 46.78 GeV. In the μ+μ− and τ+τ− channels the total cross sections and the forward-backward asymmetries are reported and compared with other experiments. The results are in excellent agreement with the standard model. The weak-neutral-current vector and axial-vector coupling constants are determined. The values for muons and τ’s are compatible with universality and with the predictions of the standard model. In the μ+μ−γ channel, all measured distributions, including the forward-backward muon asymmetry, are in excellent agreement with the electroweak theory. Our data on the two-photon process, e+e−μ+μ−, agrees with QED to order α4 over the entire energy range and the Q2 range from 0.7 to 166 GeV2.
SIG(QED) = 86.85/S.
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A study of τ-lepton production in the CMS energy region from 14 to 46.8 GeV at PETRA is reported. The cross section, the decay branching ratio into μν ν , and the electroweak parameters are determined with a total integrated luminosity of 115 pb −1 .
Total cross section calculated from measured channel cross section assuming BR(tau-mu) = 17.6 pct. SIG(Q=MU) is the QED point cross section.
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We use the reaction e+e−→μ+μ−, in the Mark J detector at the DESY high-energy e+e− collider PETRA, to test the standard electroweak theory and find good agreement. We also set limits on the parameters of several extended gauge theories.
CROSS SECTION MEASUREMENT RELATIVE TO PREDICTED QED CROSS SECTION.
FORWARD-BACKWARD ASYMMETRY. THE SYSTEMATIC ERROR IN THE ASYMMETRY IS <0.5 PCT.
ANGULAR DISTRIBUTIONS NOT GIVEN IN PAPER. SUPPLIED BY E.DEFFUR.
We have measured, at an average centre-of-mass energy of 34.22 GeV a forward-backward charge asymmetry in the reaction e + e − → μ + μ − of value −0.161 ± 0.032. This demonstrates the existence of an axial vector neutral current with coupling strength of g e a g μ a =0.53 ± 0.10. We have also obtained a limit on the vector coupling strength of g e v g μ v <0.12. The Weinberg angle is found to be sin 2 θ W =0.29 +0.09 −0.11 . From the reaction e + e − → τ + τ − we have found g e a g τ a <0.34, g e v g τ v <0.55.
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We have measured the process e+e−→μ+μ− at √s =29 GeV using the High Resolution Spectrometer at SLAC PEP. The forward-backward charge asymmetry is Aμμ=-(4.9±1.5±0.5)% based on 5057 events. A subsample of 3488 μ+μ− events in the angular range ‖cosθ‖<0.55 gives a cross-section ratio of Rμμ=0.990±0.017±0.030. The resulting couplings of the weak neutral current are gaegaμ=0.208±0.064± 0.021 and gvegvμ=0.027 ±0.051±0.089. The QED cutoff parameters are Λ+>170 GeV and Λ−>146 GeV at 95% C.L.
Corrected for acceptance and O(alpha**3) QED radiation. Numerical values taken from SUGANO-ANL-HEP-CP-84-90.
Forward-backward asymmetry based on fit to angular distribution. Result is given combined with earlier data from BENDER et al.
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A high-precision measurement of the differential cross section for Bhabha scattering (e+e−→e+e−) is presented. The measurement was performed with the MAC detector at the PEP storage ring of the Stanford Linear Accelerator Center, at a center-of-mass energy of 29 GeV. Effects due to electroweak interference are observed and agree well with the predictions of the Glashow-Salam-Weinberg model. The agreement between the data and the electroweak prediction rules out substructure of the electron up to mass scales of 1 TeV.
Error contains both statistics and systematics.
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We present measurements of forward-backward energy asymmetries of τ-lepton decay products from the reaction e+e−→τ+τ− in data collected with the MAC detector operating at the SLAC storage ring PEP at a center-of-mass energy of 29 GeV. The energy asymmetries for the decays τ→ντeν¯e, τ→ντμν¯μ, τ→ντπ, and τ→ντρ are interpreted as effects caused by the combination of maximally parity-violating weak τ decays and a longitudinal polarization produced by the interference of electromagnetic and weak processes. From the forward-backward polarization asymmetry AP=(0.06±0.07)×(1±0.011), we determine the coupling-constant product gaegvτ=(0.26 ±0.31)×(1±0.011). Assuming gae=-(1/2 as expected, we find gvτ=(-0.52±0.62)×(1±0.011), consistent with the prediction of the Glashow-Weinberg-Salam model of electroweak interactions. Alternatively, assuming the standard-model prediction of negligible polarization in τ-pair production, the leptonic energy spectra are used to measure the Michel parameter to be 0.79±0.10±0.10, consistent with the V-A hypothesis for the τν¯τ-W vertex.
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A high-statistics measurement has been made of the process e+e−→μ+μ− at s=29 GeV with the MAC detector at the SLAC storage ring PEP. The electroweak forward-backward charge asymmetry for a sample of approximately 16 000 events was measured to be Aμμ=−0.063±0.008±0.002. The ratio of the cross section to the lowest-order QED cross section was measured to be Rμμ=1.01±0.01±0.03. From these results the weak neutral axial-vector and vector couplings are determined to be gAegAμ=0.25±0.03±0.01 and gVegVμ=−0.02±0.03±0.09.
Data are fully corrected, including radiative effects.
Asymmetry determined from a two parameter fit to the angular distribution proportional to R*(1 + cos(theta)**2 + (8/3)*A*cos(theta)). R is then the total ratio relative to the lowest order QED cross section and A is the forward-backward asymmetry.
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A high-statistics measurement is presented of the cross section for the process e+e−→τ+τ− at s=29 GeV from the MAC detector at PEP. A fit to the angular distribution of our sample of 10 153 events with |cosθ|<0.9 gives an asymmetry Aττ=−0.055±0.012±0.005 from which we find the product of electron and tau axial-vector weak neutral couplings gAegAτ=0.22±0.05.
Data fully corrected up to O(ALPHA**3) radiative effects. Data requested from authors.
Data extrapolated to full acceptance.
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The differential cross sections for lepton pair production in e+e− annihilation at 29 GeV have been measured and found to be in good agreement with the standard model of the electroweak interaction. With the assumption of e−μ−τ universality, the weak neutral-current couplings are determined to be ga2=0.23±0.05 and gv2=0.03±0.04.
Numerical values supplied by M.Levi.
Data requested from authors.
Extrapolated to full angular range.