Measurement of the running of the electromagnetic coupling at large momentum-transfer at LEP

The evolution of the electromagnetic coupling, alpha, in the momentum-transfer range 1800 GeV2 < -Q(2) < 21600 GeV2 is studied with about 40 000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies root s = 189-209 GeV. The running of a is parametrised as: alpha(Q(2)) = (1-C Delta alpha(Q2))/(alpha 0) where alpha(0) = alpha(Q(2) = 0) is the fine-structure constant and C = 1 corresponds to the evolution expected in QED. A fit to the differential cross section of the e(+)e(-) -> e(+)e(-) process for scattering angles in the range vertical bar cos theta vertical bar < 0.9 excludes the hypothesis of a constant value of alpha, C = 0, and validates the QED prediction with the result: C = 1.05 +/- 0.07 +/- 0.14, where the first uncertainty is statistical and the second systematic. (c) 2005 Elsevier B.V. All rights reserved.