We search for lepton-flavor-violating ${\tau }^{-}\to e^{-}\alpha$ and ${\tau }^{-}\to {\mu }^{-}\alpha$ 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 $62.8{\mathrm{fb}}^{-1}$, 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 ${\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau }$ and ${\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio $\mathcal{B}({\tau }^{-}\to e^{-}\alpha )/\mathcal{B}({\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau })$ in the range $(1.1-9.7)\times {10}^{-3}$ and on $\mathcal{B}({\tau }^{-}\to {\mu }^{-}\alpha )/\mathcal{B}({\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau })$ in the range $(0.7-12.2)\times {10}^{-3}$ for $\alpha$ masses between 0 and $1.6\mathrm{GeV}/c^2$. These results provide the most stringent bounds on invisible boson production from $\tau$ decays.

• Received 7 December 2022
• Accepted 23 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.181803

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Published by the American Physical Society