Search for lepton flavour violating decay $\tau^- \to \mu^+ \mu^{\mathstrut -} \mu^{\mathstrut -}$ decays in Belle II data

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.
Tags
Working group Tau
Principal authors @  Robin Leboucher, Alberto Martini, Armine Rostomyan, Justine Serrano, Laura Zani
Reference JHEP 09, 062 (2024), DOI: 10.1007/JHEP09(2024)062
Document BELLE2-PUB-PH-2023-003
Links arXiv:2405.07386, Inspire, JHEP
Bibtex 
@article{Belle-II:2024sce,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Search for lepton-flavor-violating {\ensuremath{\tau}}$^{−}${\textrightarrow} {\ensuremath{\mu}}$^{−}${\ensuremath{\mu}}$^{+}${\ensuremath{\mu}}$^{−}$ decays at Belle II}",
    eprint = "2405.07386",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    reportNumber = "Belle II Preprint 2024-012 KEK Preprint 2024-6",
    doi = "10.1007/JHEP09(2024)062",
    journal = "JHEP",
    volume = "09",
    pages = "062",
    year = "2024"
}