Observation of the semileptonic decay $B^{+}to poverline{p}mu^{+} u_{mu}$

The Cabibbo-suppressed semileptonic decay $B^{+}to poverline{p}mu^{+} u_{mu}$ is observed for the first time using a sample of $pp$ collisions corresponding to an integrated luminosity of 1.0, 2.0 and 1.7fb$^{-1}$ at centre-of-mass energies of 7, 8 and 13TeV, respectively. The differential branching fraction is measured as a function of the $poverline{p}$ invariant mass using the decay mode $B^{+}to J/psi K^{+}$ for normalisation. The total branching fraction is measured to be begin{align*} mathcal{B}(B^{+}to poverline{p}mu^{+} u_{mu}) = (5.27 ^{+0.23}_{-0.24} pm 0.21 pm 0.15)times 10^{-6}, end{align*} where the first uncertainty is statistical, the second systematic and the third is from the uncertainty on the branching fraction of the normalisation channel.