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High Energy Physics - Experiment, hep-ex
Abstract:
We report a search for lepton-flavor-violating decays τ−→ℓ−ϕ (ℓ−=e−,μ−) at the Belle II experiment, using a sample of electron-positron data produced at the SuperKEKB collider in 2019-2021 and corresponding to an integrated luminosity of 190 fb−1. We use a new untagged selection for e+e−→τ+τ− events, where the signal τ is searched for as a neutrinoless final state of a single charged lepton and a ϕ meson and the other τ is not reconstructed in any specific decay mode, in contrast to previous measurements by the BaBar and Belle experiments. We find no evidence for τ−→ℓ−ϕ decays and obtain upper limits on the branching fractions at 90% confidence level of 23 ×10−8 and 9.7×10−8 for τ−→e−ϕ and τ−→μ−ϕ, respectively