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− ̄νeντ and τ− → μ− ̄νμντ decays. We report 95% confidence-level upper limits on the branching-fraction ratio Bðτ− → e−αÞ=Bðτ− → e− ̄νeντÞ in the range ð1.1–9.7Þ × 10−3 and on Bðτ− → μ−αÞ=Bðτ− → μ− ̄νμντÞ 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.
