Published January 1, 2025 | Version v1
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Search for lepton-flavor-violating τ- → l-Ks0 decays at Belle and Belle II

Description

We present the results of a search for charged-lepton-flavor violating decays tau- -> l- K0S,1 where l- 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- -> tau+ tau- events. We observe 0 and 1 events and set 90% confidence level upper limits of 0.8x10-8 and 1.2x10-8 on the branching fractions of the decay modes tau- -> e- K0S and tau- -> mu-K0S, respectively. These are the most stringent upper limits to date.

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