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

TL;DR

This paper reports the first clear evidence of the running of the QED coupling alpha(t) in the space-like region from small-angle Bhabha scattering at LEP, confirming theoretical predictions and observing hadronic contributions directly.

Contribution

It provides the most significant direct measurement of alpha(t) running in a single experiment, including the first observation of hadronic effects in this region.

Findings

Measured alpha(t) for 2 ≤ -t ≤ 6 GeV^2

Confirmed agreement with theoretical evaluations

First direct evidence of hadronic contribution

Abstract

Using the OPAL detector at LEP, the running of the effective QED coupling alpha(t) is measured for space-like momentum transfer, 2 \leq -t \leq 6 GeV^2, from the angular distribution of small-angle Bhabha scattering. This is currently the most significant direct observation of the running of the QED coupling in a single experiment and the first clear evidence of the hadronic contribution to the running in the space-like region. Our result is in good agreement with standard evaluations of alpha(t), based on data in the time-like region.