Muon decay spin asymmetry

We compute the spin asymmetry of the muon decay μ→eaccent="true">ν¯eν_μ through O(α²) in perturbative QED. These two-loop corrections are about a factor 5 (20) smaller than the current statistical (systematic) uncertainty of the most precise measurement, performed by the TWIST Collaboration. We point out that at O(α²) the asymmetry requires a careful definition due to multilepton final states, and we suggest to use familiar QCD techniques to define it in an infrared safe way. We find that the TWIST measurement of the asymmetry is in excellent agreement with the Standard Model.