Search for lepton-flavor-violating $τ^- \to l^- K_S^0$ decays at Belle and Belle II

Abstract We present the results of a search for charged-lepton-flavor violating decays $\tau^{\pm} \rightarrow l^{\pm}K_{S}^{0}$, where $l^{\pm}$ is either an electron or a muon produced in association with a neutral $K_S^0$ meson. We use a data set of $e^+e^-$ collisions composed of a 424\invfb sample of data recorded by the Belle II experiment and a $980~$fb$^{-1}$ sample of data recorded by the Belle experiment. We observe XX events and set a $90\%$ confidence level upper limit of $XX \times 10^{-8}$ and $YY \times 10^{-8}$ on the branching fraction of $\tau^{\pm} \rightarrow e^{\pm}K_{S}^{0}$ and $\tau^{\pm} \rightarrow \mu^{\pm}K_{S}^{0}$ decays, respectively.
Tags Belle Run 1 Run 2 (partial)
Working group Tau
Principal authors @  Klemens Lautenbach, Justine Serrano, Laura Zani
Reference JHEP 08, 092 (2025), DOI: 10.1007/JHEP08(2025)092
Document BELLE2-PUB-DRAFT-2024-006
Links arXiv:2504.15745, Inspire, JHEP
Bibtex 
@article{Belle:2025iff,
    author = "Adachi, I. and others",
    collaboration = "Belle, Belle-II",
    title = "{Search for lepton-flavor-violating ${\tau }^{-}\to {{\ell}}^{-}{K}_{s}^{0}$ decays at Belle and Belle II}",
    eprint = "2504.15745",
    archivePrefix = "arXiv",
    primaryClass = "hep-ex",
    doi = "10.1007/JHEP08(2025)092",
    journal = "JHEP",
    volume = "08",
    pages = "092",
    year = "2025"
}