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First observation of the decay $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$

A discovery of the rare decay $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$ is presented. This decay is observed for the first time, with 5.2 $σ$ significance. The observation is made using $pp$ collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector. The measured branching fraction is (2.3 $\pm$ 0.6 (stat.) $\pm$ 0.1 (syst.))$\times 10^{-8}$, and the ratio of the $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$ and $B^{+} \rightarrow \K^{+} μ^{+} μ^{-}$ branching fractions is measured to be 0.053 $\pm$ 0.014 (stat.) $\pm$ 0.001 (syst.).