Search for Lepton-Flavor-Violating τ Decays to a Lepton and an Invisible Boson at Belle II

We search for lepton-flavor-violating τ− → e− α and τ− → μ− α decays, where α is an invisible spin-0 boson. The search uses electron-positron collisions at 10.58 GeV center-of-mass energy with an integrated luminosity of 62.8 fb−1 , produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known τ− → e− ν̄e ντ and τ− → μ− ν̄μ ντ decays. We report 95% confidence-level upper limits on the branching-fraction ratio Bðτ− → e− αÞ=Bðτ− → e− ν̄e ντ Þ in the range ð1.1–9.7Þ × 10−3 and on Bðτ− → μ− αÞ=Bðτ− → μ− ν̄μ ντ Þ in the range ð0.7–12.2Þ × 10−3 for α masses between 0 and 1.6 GeV=c2 . These results provide the most stringent bounds on invisible boson production from τ decays.