Search for lepton-flavor-violating $${\tau }^{-}\to {{\ell}}^{-}{K}_{s}^{0}$$ decays at Belle and Belle II

We present the results of a search for charged-lepton-flavor violating decays, 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 and, respectively. These are the most stringent upper limits to date.