Search for the rare decay $B^{+}\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\nu }_{\mu }$

A search for the rare leptonic decay $B^{+} \rightarrow {\mu}^{+}{\mu}^{-}{\mu}^{+}{\nu}_{{\mu}}$ is performed using proton-proton collision data corresponding to an integrated luminosity of $4.7$ fb${}^{-1}$ collected by the LHCb experiment. The search is carried out in the region where the lowest of the two ${\mu }^{+}{\mu }^{-}$ mass combinations is below $980$MeV/c${}^2$. The data are consistent with the background-only hypothesis and an upper limit of $1.6\times {10}^{-8}$ at 95% confidence level is set on the branching fraction in the stated kinematic region.