# The 16th International Workshop on Tau Lepton Physics (TAU2021) (Virtual Edition)

September 27, 2021 to October 1, 2021
Indiana University
America/Indiana/Indianapolis timezone

## 𝜏→𝜇𝜇𝜇 at a rate of one out of 10^14 tau decays?

Sep 27, 2021, 1:40 PM
20m
Virtual (Indiana University)

### Virtual

#### Indiana University

Oral contribution Tau2021 Abstracts

### Speaker

Patrick Blackstone (Indiana University)

### Description

We present in full analytic form the partial widths for the lepton flavor violating decays $L^\pm \to \ell^\pm \ell'^{+} \ell'^{-}$, with $L = \tau, \mu$ and $\ell^{(\prime)} = \mu, e$, mediated by neutrino oscillations in the one-loop diagrams. Compared to the first result by Petcov:1976ff, which was obtained in the nonphysical zero momentum limit $\mathcal{P} \ll m_{\nu} \ll M_W$, we retain full dependence on external scales $\mathcal{P}$ and determine the branching ratios in the physical limit $m_\nu \ll \mathcal{P} \ll M_W$. We show that in this limit the conclusion by Pham:1998fq that $\tau \to \ell \ell' \ell'$ could be as large as $10^{-14}$ is flawed. In this talk we will describe the details of our calculation, present our results, and motivate some of the peculiarities of this calculation from the viewpoint of effective field theory.

What is your topic? Lepton universality and flavour violation

### Primary authors

Patrick Blackstone (Indiana University) Matteo Fael (KIT Karlsruhe) Emilie Passemar (Indiana University)

### Presentation materials

 Tau 2021 Blackstone Presentation.pdf VideoTau2021_Blackstone.mp4