The electron energy spectrum in muon decay through O;(α²)

We compute the complete (α²) QED corrections to the electron energy spectrum in unpolarized muon decay, including the full dependence on the electron mass. Our calculation reduces the theoretical uncertainty on the electron energy spectrum well below 10^-4, the precision anticipated by the TWIST experiment at TRIUMF, which is currently performing this measurement. For this calculation, we extend techniques we have recently developed for performing next-to-next-to-leading order computations to handle the decay spectra of massive particles. Such an extension enables further applications to precision predictions for b, t, and Higgs differential decay rates.