Observation of the suppressed decay Λ_b ⁰ → pπ ⁻ μ ⁺ μ ⁻

The suppressed decay Λ_b ⁰ → 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 Λ_b ⁰ → pπ⁻μ⁺μ⁻ branching fraction is measured relative to the Λ_b ⁰ → 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.]