JHEP 2024, (2024)

ISSN: 1029-8479, SCIE

Search for lepton-flavor-violating τ − → µ −µ +µ − decays at Belle II

Belle II Collaboration (I. Adachi, ..., N. Anh Ky, ...)

We present the result of a search for the charged-lepton-flavor violating decay τ − → µ −µ +µ − using a 424 fb−1 sample of data recorded by the Belle II experiment at the SuperKEKB e +e − collider. The selection of e +e − → τ +τ − 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 1.9×10−8 on the branching fraction of the τ − → µ −µ +µ − decay, which is the most stringent bound to date.

DOI: https://doi.org/10.48550/arXiv.2405.07386

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