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

We search for lepton-flavor-violating tau- -> e-alpha and tau- -> mu-alpha decays, where alpha 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 tau- -> e- nu over bar e nu tau and tau- -> mu- nu over bar mu nu tau decays. We report 95% confidence-level upper limits on the branching-fraction ratio Bo tau- -> e-alpha thorn =Bo tau- -> e-nu over bar e nu tau thorn in the range o1.1-9.7 thorn x 10-3 and on Bo tau- -> mu- alpha thorn =Bo tau- -> mu- nu over bar mu nu tau thorn in the range o0.7-12.2 thorn x 10-3 for alpha masses between 0 and 1.6 GeV=c2. These results provide the most stringent bounds on invisible boson production from tau decays.