Search for lepton-flavor-violating $\tau^- \to \mu^-\mu^+\mu^-$ decays at Belle II

TL;DR

This paper reports a search for the lepton-flavor-violating decay of tau into three muons at Belle II, setting the most stringent upper limit to date on its branching fraction, with one candidate observed consistent with background.

Contribution

First search for $ au^- o \mu^-\mu^+\mu^-$ at Belle II, establishing the tightest upper limit on its branching ratio to date.

Findings

Observed one candidate event consistent with background.

Set a 90% CL upper limit of 1.9 x 10^{-8} on the branching fraction.

Most stringent bound on this decay channel to date.

Abstract

We present the result of a search for the charged-lepton-flavor violating decay $\tau^- \to \mu^-\mu^+\mu^-$ using a $424fb^{-1}$ sample of data recorded by the Belle II experiment at the SuperKEKB $e^{-}e^{+}$ collider. The selection of $e^{-}e^{+}\to\tau^+\tau^-$ events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a $90\%$ confidence level upper limit of $1.9 \times 10^{-8}$ on the branching fraction of the \taumu decay, which is the most stringent bound to date.