Observation of the suppressed decay Λb0 → pπ− μ+ μ−

The suppressed decay Λ0b → 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 Λ0b → pπ − μ + μ − branching fraction is measured relative to the Λ0b → J/ψ(→μ + μ −)pπ − branching fraction giving B(Λ0b→pπ−μ+μ−)B(Λ0b→J/ψ(→μ+μ−)pπ−)=0.044±0.012±0.007,B(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.