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

TL;DR

This paper reports a precise measurement of the running of the QED coupling constant alpha(t) in small-angle Bhabha scattering at LEP, confirming its variation with momentum transfer and excluding constant alpha hypotheses.

Contribution

First direct differential measurement of alpha(t) running in the space-like region at LEP, with high significance and agreement with theoretical predictions.

Findings

Alpha(t) runs as predicted by QED.

Constant alpha hypothesis rejected at over 5 sigma.

Leptonic loop only contribution rejected at 3 sigma.

Abstract

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(-6.07GeV^2) - Delta alpha(-1.81GeV^2) = (440 pm 58 pm 43 pm 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 5sigma. Similarly, our results are inconsistent at the level of 3sigma with the hypothesis that only leptonic loops contribute to the running. This is currently the most significant direct measurment where the running alpha(t) is probed differentially within the measured t range.