Muon decay spin asymmetry

Abstract

We compute the spin asymmetry of the muon decay through O(alpha^2) in perturbative QED. These two-loop corrections are about a factor five (twenty) smaller than the current statistical (systematic) uncertainty of the most precise measurement, performed by the TWIST collaboration. We point out that at O(alpha^2) the asymmetry requires a careful definition due to multi-lepton final states and suggest to use familiar QCD techniques to define it in an infra-red safe way. We find that the TWIST measurement of the asymmetry is in excellent agreement with the Standard Model.

arXiv comment5 pages, 2 figures

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• Michel parameters in the presence of massive Dirac and Majorana neutrinos
• Radiative (anti)neutrino energy spectra from muon, pion, and kaon decays
• Sector-improved residue subtraction: Improvements and Applications
• On spin asymmetry in muon and tau decays