We have measured the cross section for γγ production with the TOPAZ detector in the energy region √ s =50.0–61.4 GeV. The observed cross section for γγ production integrated over |cos θ | ⩽ 0.77 is 50.2±0.8±2.2 pb at 〈√ s 〉=57.6 GeV and the ratio of this value to the QED prediction is 1.01±0.02±0.04. The angular distribution is in good agreement with the QED predictions, thereby setting limits on the compositeness scales, Λ L+R + =168 GeV, Λ L+R − =97 GeV, Λ L,R =141 GeV, Λ L,R − =81 GeV, and Λ L−R ± =68 GeV, at the 95% confidence level. The reaction e + e − → γγγ was also studied and was found consistent with the QED prediction.
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The total and differential cross-sections for the reaction e + e − → γγ ( γ ) are measured at centre of mass energies around 91 GeV using an integrated luminosity of 4.7 pb −1 . The aggreement with QED prediction is good. Consequently there is no evidence for non-standard channels which would have the same experimental signature. The lower limits on the QED cuttoff parameters are Λ + > 113 GeV and Λ − > 95 GeV. An upper limit on the effective coupling between a possible excited electron and the gamma is derived. At 95% confidence level the branching ratios for Z 0 decay into π 0 γ, ηψ and γγγ are below 1.5 × 10 −4 , 2.8 × 10 −4 and 1.4 × 10 −4 respectively.
Radiative effects are subtracted.
Radiative effects subtracted.
The charge asymmetry of quark jets produced in e + e − annihilations at 〈√ s 〉=57.9 GeV was measured with the TOPAZ detector at TRISTAN. The observed charge asymmetry is +0.091±0.014(stat.)±0.016(sys.). From the measured differential cross section, the axial vector coupling constant averaged over all quark flavors was determined to be 1.09 −0.21 +0.27 . These values are consistent with the standard model predictions. Possible deviations from the standard model were examined in terms of contact interactions, and the lower limits on the compositeness scale parameters were obtained to be 1.2–7.1 TeV at the 95% confidence level.
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The reaction p + d → π − + X has been studied at Saturne at 1.45, 2.10 and 2.70 GeV, using the spectrometer SPES III. The analysing power and the differential cross sections show no evidence for narrow structures which could be interpreted as a resonance in the three baryon system. At the lowest incident energy, the shapes of the experimental cross sections are well reproduced by phase-space distributions.
Axis error includes +- 0.0/0.0 contribution (?////).
The cross section of the pure QED process e + e − → γγ has been measured using data accumulated during the 1989 and 1990 scans of the Z 0 resonance at LEP. Both the energy dependence and the angular distribution are in good agreement with the QED prediction. Upper limits on the branching ratios of Z 0 → γγ , Z 0 → π 0 γ and Z 0 → ηγ have been set at 1.4×10 −4 , 1.4×10 −4 and 2.0×10 −4 respectively. Lower limits on the cutoff parameters of the modified electron propagator have been found to be Λ + > 117 GeV and Λ − > 110 GeV. The reaction e + e − → γγγ has also been studied and was found to be consistent with the QED prediction. An upper limit on the branching ratio of Z 0 → γγγ has been set at 6.6 × 10 −5 . All the limits are given at 95% confidence level.
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Differential cross section data of the CELLO experiment on pair production of muons, taus, and heavy quarks ine+e−-annihilation are presented and analysed, together with our data on Bhabha scattering, in terms of compositeness effects characterized by the mass scale Λ. We discuss difficulties in the combination of limits Λ from different experiments. The appropriate parameter to combine different results turns out to be ɛ=±1/Λ2, which is in contrast to Λ Gaussian distributed.
Errors are combined statistics and systematics.
Errors are combined statistics and systematics.
Errors are combined statistics and systematics.
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Axis error includes +- 0.0/0.0 contribution (?////).
Axis error includes +- 0.0/0.0 contribution (?////).
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The pure QED reaction e + e − → γγ has been studied at centre of mass energies around the mass of the Z 0 boson using data recorded by the OPAL detector at LEP. The results are in good agreement with the QED prediction. Lower limits on the cutoff parameters of the modified electron propagator are found to be Λ + >89 GeV and Λ. The lower limit on the mass of an excited electron is 82 GeV assuming the coupling constant λ =1. Upper limits on the branching ratios of Z 0 → γγ , Z 0 → π 0 γ and Z 0 → ηγ are set at 3.7×10 −4 , 3.9×10 −4 and 5.8×10 −4 respectively. Two events from the reaction e + e − → γγγ have been observed, consistent with the QED prediction. An upper limit on the branching ratio of Z 0 → γγγ is set at 2.8×10 −4 . All the limits are given at 95% confidence level.
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Data read from graph.
The reaction e + e − → τ + τ − has been studied at centre of mass energies between 14.0 and 46.8 GeV with the CELLO detector at the PETRA e + e − collider. We present results for the cross section σ τ and the charge asymmetry A τ . The results are in good agreement with the standard model. We have also measured the topological decay rates BR 1 , BR 3 and BR 5 for the inclusive decay of the τ lepton into one, three and five charge particles. The results confirm and improve earlier CELLO measurements at other energies. We find for the combined values at all energies BR 1 = (84.9 ± 0.4 ± 0.3)%, BR 3 = (15.0 ± 0.4 ± 0.3)% and BR 5 = (0.16 ± 0.13 ± 0.04)%.
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Corrected for radiative effects and background contributions.
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