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[References: 42 tit.]

The Cabibbo-suppressed semileptonic decay B+→pp¯¯¯μ+νμB+→pp¯μ+νμ is observed for the first time using a sample of pp collisions corresponding to an integrated luminosity of 1.0, 2.0 and 1.7 fb−1 at centre-of-mass energies of 7, 8 and 13 TeV, respectively. The differential branching fraction is measured as a function of the pp¯¯¯pp¯ invariant mass using the decay mode B+→ J/ψK+ for normalisation. The total branching fraction is measured to beB(B+→pp¯¯¯μ+νμ)=(5.27+0.23−0.24±0.21±0.15)×10−6,B(B+→pp¯μ+νμ)=(5.27−0.24+0.23±0.21±0.15)×10−6,where the first uncertainty is statistical, the second systematic and the third is from the uncertainty on the branching fraction of the normalisation channel.