Fully differential NLO predictions for the radiative decay of muons and taus

Abstract

We present a general purpose Monte Carlo program for the calculation of the radiative muon decay μ→eνν¯γ and the radiative decays τ→eνν¯γ and τ→μνν¯γ at next-to-leading order in the Fermi theory. The full dependence on the lepton masses and polarization of the initial-sate lepton are kept. We study the branching ratios for these processes and show that fully-differential next-to-leading order corrections are important for addressing a tension between BaBar's recent measurement of the branching ratio B(τ→eνν¯γ) and the Standard Model prediction. In addition, we study various distributions of the process μ→eνν¯γ and obtain precise predictions for the irreducible background to μ→eγ searches, tailored to the geometry of the MEG detector.