Right-handed electrons in radiative muon decay

Electrons emitted in the radiative decay μ−→e−ν̄eνμγ have a significant probability of being right-handed, even in the limit me→0. Such “wrong-helicity” electrons, arising from helicity-flip bremsstrahlung, contribute an amount α4πΓ0 to the muon decay width (Γ0≡GF2mμ5/(192π3)). We use the helicity-flip splitting function Dhf(z) of Falk and Sehgal [Phys. Lett. B 325 (1994) 509] to obtain the spectrum of the right-handed electrons and the photons that accompany them. For a minimum photon energy Eγ=10 MeV (20 MeV), approximately 4% (7%) of electrons in radiative μ-decay are right-handed.