Abstract
The evolution of the electromagnetic coupling, α, in the momentum-transfer range 1800 GeV2<−Q2<21600 GeV2 is studied with about 40 000 Bhabha-scattering events collected with the L3 detector at LEP at centre-of-mass energies s=189–209 GeV. The running of α is parametrised as: α(Q2)=α01−CΔα(Q2), where α0≡α(Q2=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 |cosθ|<0.9 excludes the hypothesis of a constant value of α, 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.
Highlights
A fundam ental consequence of quantum field theory is th a t th e value of th e electrom agnetic coupling, a, depends on, or runs w ith, th e squared m om entum tran sfer, Q 2
W here N L is th e num ber of events observed in th e fiducial volum e of th e lum inosity m onitor, a L(C ) is th e corresponding e+e- ^ e+e- cross section for a given value of C and eL(C ) is the detector acceptance. This acceptance may depend on C due to the combined effect of small angular anisotropies of detector efficiencies and the dependence of the predicted differential cross section on C
The resu lt p resented above establishes th e evolution of th e electrom agnetic coupling w ith —Q 2 in th e range 1800 GeV2 < —Q 2 < 21600 GeV2
Summary
A fundam ental consequence of quantum field theory is th a t th e value of th e electrom agnetic coupling, a , depends on, or runs w ith, th e squared m om entum tran sfer, Q 2 This phenom enon is due to higher m om entum -transfers probing virtual-loop corrections to th e photon propagator. The running of a in large-angle B habha scattering was first investigated by the VENUS C o llab o ratio natTR IST A N in th e range 100 GeV2 < —Q 2 < 2916 GeV2 [12], L ater, th e L3 C ollaboration studied th e sam e process at i/ s = 189 GeV for scattering angles 0.81 < | cos#) < 0.94, p robing th e range 12.25 GeV2 < —Q 2 < 3434 GeV2 [10]. Less th a n 1% of the events scatter backwards, cos 0 < 0 , and this analysis effectively probes the region 1800 GeV2 < —Q 2 < 21600 GeV2, ex tending an d com plem enting previous space-like studies
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