$$\tau \rightarrow \mu \mu \mu $$ at a rate of one out of $$10^{14}$$ tau decays?

Blackstone, Patrick; Fael, Matteo; Passemar, Emilie

doi:10.1140/epjc/s10052-020-8059-7

Abstract A search for the very rare$$B^{*0}\rightarrow \mu ^+\mu ^-$$

B^{* 0} \to μ^{+} μ^{-}

and$$B_{s}^{*0}\rightarrow \mu ^+\mu ^-$$

B_{s}^{* 0} \to μ^{+} μ^{-}

decays is conducted by analysing the$$B_c^+\rightarrow \pi ^+\mu ^+\mu ^-$$

B_{c}^{+} \to π^{+} μ^{+} μ^{-}

process. The analysis uses proton-proton collision data collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9$$\text {\,fb}^{-1}$$

{\,fb}^{- 1}

. The signal signatures correspond to simultaneous peaks in the$$\mu ^+\mu ^-$$

μ^{+} μ^{-}

and$$\pi ^+\mu ^+\mu ^-$$

π^{+} μ^{+} μ^{-}

invariant masses. No evidence for an excess of events over background is observed for either signal decay mode. Upper limits at the$$90\%$$

90 %

confidence level are set on the branching fractions relative to that for$$B_c^+\rightarrow J\hspace{-1.66656pt}/\hspace{-1.111pt}\psi \pi ^+$$

B_{c}^{+} \to J / ψ π^{+}

decays,$$\begin{aligned} \mathcal{R}_{B^{*0}(\mu ^+\mu ^-)\pi ^+/J\hspace{-1.66656pt}/\hspace{-1.111pt}\psi \pi ^+}&< 3.8\times 10^{-5}\ \text { and }\\ \mathcal{R}_{B_{s}^{*0}(\mu ^+\mu ^-)\pi ^+/J\hspace{-1.66656pt}/\hspace{-1.111pt}\psi \pi ^+}&< 5.0\times 10^{-5}. \end{aligned}$$

\begin{matrix} R_{B^{* 0} (μ^{+} μ^{-}) π^{+} / J / ψ π^{+}} & < 3.8 \times 10^{- 5} and \\ R_{B_{s}^{* 0} (μ^{+} μ^{-}) π^{+} / J / ψ π^{+}} & < 5.0 \times 10^{- 5} . \end{matrix}

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