Measurement of the running of the QED coupling in small angle Bhabha scattering with the OPAL detector

Using the high precision OPAL Silicon-Tungsten luminometer at LEP, the running of the effective QED coupling α(t) is measured for space-like momentum transfer 1.81 ≤ -t ≤ 6.07 GeV2 through its effect on the angular spectrum of small angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain a strong direct evidence that the running of α(t) is consistent with standard model expectations. The null hypothesis that α remains constant within the above interval of -t is excluded with a significance above 5σ: Δα(-6.07 GeV2) - Δα(-1.81 GeV2) = 0.00450 ± 0.00079 The hadronic contribution to the running of the coupling has been estimated to be: Δαhad(-6.07 GeV2) - Δαhad(-1.81 GeV2) = 0.00248 ± 0.00079. This result is inconsistent at the level of more than 3σ with the hypothesis that only leptonic loops contribute to the running, and therefore provide the first clear space-like experimental evidence that hadronic loops also contribute. (orig.)