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

Abstract

We present the result of a search for the charged-lepton-flavor violating decays ${\tau }^{-}\to e^{\mp }{\ell }^{\pm }{\ell }^{-}$, where $\ell$ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb${}^{-1}$ recorded by the Belle II experiment at the SuperKEKB $e^{+}{e}^{-}$ collider. The selection of $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 $\times 10^{-8}$ are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes.

22 pages, 9 figures. arXiv admin note: text overlap with arXiv:2405.07386

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