Search for the lepton-flavor-violating $\tau^- \rightarrow e^\mp \ell^\pm \ell^-$ decays at Belle II

Abstract We present the result of a search for the charged-lepton-flavor violating decays $\tau^- \rightarrow e^\mp \ell^\pm \ell^-$, where $\ell$ is a muon or an electron using a 424 fb$^-1$ sample of data recorded by the Belle II experiment at the SuperKEKB \epem 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. The signal yields, obtained from an unbinned maximum likelihood fit, are compatible with the background expectation and limits are set at 90% confidence level at XX $10^{-8}$
Tags Run 1
Working group Tau
Principal authors @  Giampiero Mancinelli, Justine Serrano, Arthur Thaller, Laura Zani
Reference JHEP 12, 169 (2025), DOI: https://doi.org/10.1007/JHEP12(2025)169
Document BELLE2-PUB-DRAFT-2024-021
Links arXiv:2507.18236, Inspire, JHEP
Bibtex 
@article{Belle-II:2025urb,
    author = "Adachi, I. and others",
    collaboration = "Belle-II",
    title = "{Search for the lepton-flavor-violating $τ^{-} \rightarrow e^{\mp} \ell^{\pm} \ell^{\mp}$ decays at Belle II}",
    eprint = "2507.18236",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    doi = "10.1007/JHEP12(2025)169",
    journal = "JHEP",
    volume = "12",
    pages = "169",
    year = "2025"
}