Search for lepton-flavor-violating τ ⁻→ ⁻Kₛ⁰ decays at Belle and Belle II

A bstract We present the results of a search for charged-lepton-flavor violating decays τ ⁻→ ⁻Kₛ⁰ , 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 τ ⁻→ e⁻KS⁰ and τ ⁻→ μ ⁻KS⁰ , respectively. These are the most stringent upper limits to date.