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-ν¯eντ and τ-→μ-ν¯μντ decays. We report 95% confidence-level upper limits on the branching-fraction ratio B(τ-→e-α)/B(τ-→e-ν¯eντ) in the range (1.1-9.7)×10-3 and on B(τ-→μ-α)/B(τ-→μ-ν¯μντ) in the range (0.7-12.2)×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.