Precise measurement of Bhabha scattering at a center-of-mass energy of 57.77 GeV

Abstract

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 |cos\ensuremath{\theta}|\ensuremath{\leqslant}0.743 and 0.822\ensuremath{\leqslant}cos\ensuremath{\theta}\ensuremath{\leqslant}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 ${\mathit{q}}^{2}$ 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 ${\mathit{Z}}^{0}$ 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.