From analysis of data produced by the BABAR experiment, the first upper bounds (90% C.L.) were obtained on the branching ratios $\mathrm{Br}(\tau \to e\gamma \gamma )<2.5\times {10}^{-4}$ and $\mathrm{Br}(\tau \to \mu \gamma \gamma )<5.8\times {10}^{-4}$. In addition, improved upper bounds (95% C.L.) were found on branching ratios $\mathrm{Br}(\tau \to eX)<1.4\times {10}^{-3}$ and $\mathrm{Br}(\tau \to \mu X)<2.0\times {10}^{-3}$, where $X$ is an undetected weakly interacting boson with mass $m_X<1.6\mathrm{GeV}/c^2$.

• Received 1 June 2021
• Accepted 22 September 2021

DOI:https://doi.org/10.1103/PhysRevD.104.075032

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Published by the American Physical Society