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Using the OPAL detector at LEP, the running of the effective QED couplingalpha(t) is measured for space-like momentum transfer from the angulardistribution of small-angle Bhabha scattering. In an almost ideal QEDframework, with very favourable experimental conditions, we obtain:  Delta alpha(-6.07GeV^2) - Delta alpha(-1.81GeV^2) = (440 pm 58 pm 43 pm 30) X10^-5, where the first error is statistical, the second is the experimentalsystematic and the third is the theoretical uncertainty. This agrees withcurrent evaluations of alpha(t).The null hypothesis that alpha remains constantwithin the above interval of -t is excluded with a significance above 5sigma.Similarly, our results are inconsistent at the level of 3sigma with thehypothesis that only leptonic loops contribute to the running. This iscurrently the most significant direct measurment where the running alpha(t) isprobed differentially within the measured t range.

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