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 $\mu\to e\,\nu \bar{\nu}\gamma$ and the radiative decays $\tau\to e\, \nu \bar{\nu}\gamma$ and $\tau\to\mu\,\nu \bar{\nu}\gamma$ 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 $\mathcal{B}(\tau\to e\, \nu\bar{\nu}\gamma)$ and the Standard Model prediction. In addition, we study various distributions of the process $\mu\to e\,\nu \bar{\nu}\gamma$ and obtain precise predictions for the irreducible background to $\mu\to e \gamma$ searches, tailored to the geometry of the MEG detector.