Search for $\tau^-\to \mu^-\mu^+\mu^-$ decays at the LHCb experiment with Run 2 data

TL;DR

This paper reports a search for the lepton-flavour-violating decay $ au^- o \mu^-\mu^+\mu^-$ using LHCb Run 2 data, setting new upper limits on its branching fraction at 90% and 95% confidence levels.

Contribution

The study provides the first stringent upper limits on the branching fraction of $ au^- o \mu^-\mu^+\mu^-$ decay using LHCb Run 2 data, improving constraints on lepton-flavour violation.

Findings

Upper limit of $1.9\times 10^{-8}$ at 90% CL

Upper limit of $2.3\times 10^{-8}$ at 95% CL

No evidence of the decay observed

Abstract

A search for the lepton-flavour-violating decay $\tau^-\to \mu^-\mu^+\mu^-$ is carried out using data collected by the LHCb experiment between 2016 and 2018 in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb$^{-1}$. An upper limit of $1.9\,(2.3)\times 10^{-8}$ is set at the 90% (95%) confidence level on the branching fraction of the decay.