Bhabha scattering at a center-of-mass energy of 57.77 GeV has been measured using the VENUS detector at KEK TRISTAN. The precision is better than 1% in scattering angle regions of |cosθ|⩽0.743 and 0.822⩽cosθ⩽0.968. A model-independent scattering-angle distribution is extracted from the measurement. The distribution is in good agreement with the prediction of the standard electroweak theory. The sensitivity to underlying theories is examined, after unfolding the photon-radiation effect. The q2 dependence of the photon vacuum polarization, frequently interpreted as a running of the QED fine-structure constant, is directly observed with a significance of three standard deviations. The Z0 exchange effect is clearly seen when the distribution is compared with the prediction from QED (photon exchanges only). The agreement with the standard theory leads us to constraints on extensions of the standard theory. In all quantitative discussions, correlations in the systematic error between angular bins are taken into account by employing an error matrix technique.
Cross section is integrated over the cos(theta ) bin.
The angular distributions of the reactione+e−→μ+μ− ande+e+→τ+τ− have been measured between\(\sqrt s= 50\) and 60.8 GeV with the VENUS detector at TRISTAN. The average total cross section and the forward-backward charge asymmetry for μ-pair production are observed to be 28.3±1.4±0.8 pb and (−29.0−4.8+5.0±0.5)%, and those for τ-pair production are 27.6±1.7±1.0 pb and (−32.8−6.2+6.4±1.5)% at\(\langle \sqrt s \rangle \). These values are consistent with the predictions of the standard model of electroweak interactions.
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Differential cross sections fore+e−→γγ have been measured at center-of-mass energies of 55, 56, 56.5 and 57 GeV. The results are in good agreement with those predicted by QED. Possible deviations from QED are studied in terms of contact interactions, and the limits on compositeness scales are obtained. Using events with a gramma pair in the final state, a search is made for the pair production of unstable photons,\(e^ +e^ -\to \tilde \gamma\tilde \gamma \). No candidate events were found and a new limit on the photino mass is obtained.
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