The suppressed decay Λb0 → pπ−μ+μ−, excluding the J/ψ and ψ(2S) → μ+μ− resonances, is observed for the first time with a significance of 5.5 standard deviations. The analysis is performed with proton-proton collision data corresponding to an integrated luminosity of 3 fb−1 collected with the LHCb experiment. The Λb0 → pπ−μ+μ− branching fraction is measured relative to the Λb0 → J/ψ(→μ+μ−)pπ− branching fraction givingℬ(Λb0→pπ−μ+μ−)ℬ(Λb0→J/ψ(→μ+μ−)pπ−)=0.044±0.012±0.007, where the first uncertainty is statistical and the second is systematic. This is the first observation of a b → d transition in a baryonic decay.[Figure not available: see fulltext.]
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