Forward-region Bhabha ratios at Tera-Z could extract alpha_em(m_Z^2) with a projected statistical sensitivity of about 6e-6, below the current bottleneck.
Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP
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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.
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On the extraction of $\alpha_\textit{em}(m_Z^2)$ at Tera-$Z$
Forward-region Bhabha ratios at Tera-Z could extract alpha_em(m_Z^2) with a projected statistical sensitivity of about 6e-6, below the current bottleneck.