Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP

TL;DR

This study measures how the electromagnetic coupling constant, alpha, varies with momentum transfer at high energies, confirming QED predictions through analysis of Bhabha scattering data from LEP.

Contribution

It provides the first precise experimental validation of the running of alpha at large momentum transfer, consistent with QED expectations.

Findings

The running of alpha is observed and measured.

Results agree with QED predictions within uncertainties.

Excludes the hypothesis of a non-varying alpha at high energies.

Abstract

The evolution of the electromagnetic coupling, alpha, in the momentum-transfer range 1800GeV^2 < -Q^2 < 21600GeV^2 is studied with about 40000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies 189-209GeV. The running of alpha is parametrised as: alpha(Q^2) = alpha_0/(1-C Delta alpha(Q^2)), 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 |cos theta|<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.