Precision tests via radiative $μ$ and $τ$ leptonic decays

Abstract

The branching fractions of radiative leptonic τ decays (τ → lννγ, l = e, µ) were recently measured by the BABAR collaboration with a relative error of about 3%. The measurement of the branching ratio B(τ → eννγ), for a minimum photon energy of 10 MeV in the τ rest frame, differs from our recent SM prediction by 3.5 standard deviations, whereas our result agrees with BABAR's value for B(τ → µννγ). Our predictions also agree with the measurement of B(µ → eννγ) by the MEG collaboration. We also report on a recent proposal to test the τ dipole moments via precise measurements of radiative leptonic τ decays at high-luminosity B factories. 12th International Symposium on Radiative Corrections (Radcor 2015) and LoopFest XIV (Radiative Corrections for the LHC and Future Colliders)

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

“…After briefly discussing some technical aspects of our calculation in Section 2 and giving some details about the Monte Carlo code in Section 3 we will study the BR for the rare decays of the muon and tau in Section 4. Whenever possible we compare with previous results in the literature [23,24] and find full agreement. Making use of the flexibility of our computation, we also study the impact of the cuts used in the measurement of the BR of the tau.…”

“…Due to the photon bremsstrahlung the helicity of the final-state lepton does not have to be left-handed [18][19][20][21]. After some partial results [17,22] a full next-to-leading (NLO) calculation for the BR was presented in [23,24]. As for a related calculation of the rare decays of leptons [25], the results presented in [23,24] allow to obtain the differential decay width at NLO with cuts on the photon and electron energy and angles between them.…”

“…After some partial results [17,22] a full next-to-leading (NLO) calculation for the BR was presented in [23,24]. As for a related calculation of the rare decays of leptons [25], the results presented in [23,24] allow to obtain the differential decay width at NLO with cuts on the photon and electron energy and angles between them. In this article, we generalize these results and present a fully differential Monte Carlo program.…”

Next-to-leading order prediction for the decay μ → e e + e − ν ν ¯ $$ \mu \to e\kern0.22em \left({e}^{+}{e}^{-}\right)\;\nu \kern0.2em \overline{\nu} $$

“…Radiative muon decay, µ → eγνν, constitutes an important source of background to µ → eγ searches; secondly it provides a tool for calibration, normalization and quality check of the experiment [26]. It was studied at next-to-leading order (NLO) accuracy in [27,28]. The background in µ → eee searches originates from the accidental coincidence of normal muon-decay electrons and positrons, that within the detector resolution show the characteristics of the decay signal, and from the muon decay with internal conversion, µ → e (e + e − ) νν, which is indistinguishable from the signal except for the energy carried out by neutrinos.…”

Feasibility of τ -lepton electromagnetic dipole moments measurement using bent crystal at the LHC

“…Last two rows in the table present efficiencies of MDM and EDM reconstruction. Using latter, we can rewrite Eqs (24). and(27)in a following way…”