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October 1, 2025Open Access

Search for lepton-flavor-violating ^- ^- Kₛ⁰ decays at Belle and Belle II

Key Points

No significant events were observed, leading to stringent upper limits on branching fractions for tau decays.
The limits established are at 0.8 x 10^-8 for tau to electron decays and 1.2 x 10^-8 for tau to muon decays.
Data was combined from Belle and Belle II experiments, totaling approximately 1.3 billion tau pair events.
These results represent the most stringent limits to date on the searched lepton-flavor-violating decays.

Abstract

We present the results of a search for charged-lepton-flavor violating decays ^- ^-Kₒ^0, where ^- is either an electron or a muon. We combine e^+e^- data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb^-1) with samples recorded by the Belle experiment at the KEKB collider (980 fb^-1) to obtain a sample of 1. 3 billion e^+e^-^+^- events. We observe 0 and 1 events and set 90\% confidence level upper limits of 0. 8 10^-8 and 1. 2 10^-8 on the branching fractions of the decay modes ^- e^-Kₒ^0 and ^- ^-Kₒ^0, respectively. These are the most stringent upper limits to date.

Search for lepton-flavor-violating ^- ^- Kₛ⁰ decays at Belle and Belle II

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Abstract

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