Measurement of the running of the QED coupling in small-angle Bhabha scattering at LEP

Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer from the angular distribution of small-angle Bhabha scattering. In an almost ideal QED framework, with very favourable experimental conditions, we obtain:Delta alpha(-6.07 GeV2) - Delta alpha(-1.81 GeV2) = (440 +/- 58 +/- 43 +/- 30) x 10(-5),where the first error is statistical, the second is the experimental systematic and the third is the theoretical uncertainty. This agrees with current evaluations of alpha(t). The null hypothesis that alpha remains constant within the above interval of -t is excluded with a significance above 5 sigma. Similarly, our results are inconsistent at the level of 3 sigma with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurement where the running alpha(t) is probed differentially within the measured t range.