Leptonic $ \mu $- and $ \tau $-decays: mass effects, polarization   effects and $ O(\alpha) $ radiative corrections
  We calculate the radiative corrections to the unpolarized and the four polarized spectrum and rate functions in the leptonic decay of a polarized $ \mu $ into a polarized electron. The new feature of our calculation is that we keep the mass of the final state electron finite which is mandatory if one wants to investigate the threshold region of the decay. Analytical results are given for the energy spectrum and the polar angle distribution of the final state electron whose longitudinal and transverse polarization is calculated. We also provide analytical results on the integrated spectrum functions. We analyze the $ m_e \to 0 $ limit of our general results and investigate the quality of the $ m_e \to 0 $ approximation. In the $ m_e \to 0 $ case we discuss in some detail the role of the $ O(\alpha) $ anomalous helicity flip contribution of the final electron which survives the $ m_e \to 0 $ limit. The results presented in this 0203048 also apply to the leptonic decays of polarized $ \tau $-leptons for which we provide numerical results. 
M. Fischer, S. Groote, J.G. K\"orner and M.C. Mauser