# Physics of fundamental Symmetries and Interactions - PSI2019

Oct 20 – 25, 2019
PSI
Europe/Zurich timezone

## $\tau \to \mu \mu \mu$ at a rate of one out of $10^{14}$ tau decays?

Oct 23, 2019, 11:40 AM
20m
WHGA/001 - Auditorium (PSI)

Oral

### Speaker

Matteo Fael (KIT Karlsruhe)

### Description

Recent improvements of the Large Hadron Collider luminosity and the beginning of Belle II experiment will increase the sensitivity to tau lepton-flavour violating modes. In particular, it has been claimed by Pham in hep-ph/9810484 that $BR(\tau \to 3 \mu)$ could be as large as $10^{−14}$ in the Standard Model with massive neutrinos, contradicting earlier predictions for $BR(\mu \to 3e)$. If this were to be true, this limit could be reached experimentally in the near future (with further upgrades).

In this talk, we present a new calculation of these branching ratios in full generality within the Standard Model including massive neutrinos. By making use of the method of regions, we determine the rates as a double expansion in $m_\nu/m_\tau$ and $m_\tau/M_W$. We also report some discrepancies with previous predictions.

### Primary author

Matteo Fael (KIT Karlsruhe)

### Co-authors

Emilie Passemar (Indiana University/JLab ) Patrick Blackstone (Indiana University)