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

We present the results of a search for charged-lepton-flavor violating decays τ⁻ → ⁻KS⁰, 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⁻→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.