Search for lepton-flavor-violating ^- {}^-Kₒ^0 decays at Belle and Belle II

A bstract We present the results of a search for charged-lepton-flavor violating decays ^- {}^-Kₒ^0, 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 − → τ + τ − 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.