Observation of the suppressed decay Λ ⁰ _b  → pπ ⁻ μ ⁺ μ ⁻

Abstract

The suppressed decay Λ ⁰ _b  → 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 ⁰ _b  → pπ ⁻ μ ⁺ μ ⁻ branching fraction is measured relative to the Λ ⁰ _b  → J/ψ(→μ ⁺ μ ⁻)pπ ⁻ branching fraction giving

$$ \frac{\mathrm{\mathcal{B}}\left({\Lambda}_b^0\to p{\pi}^{-}{\mu}^{+}{\mu}^{-}\right)}{\mathrm{\mathcal{B}}\left({\Lambda}_b^0\to \mathrm{J}/\psi \left(\to {\mu}^{+}{\mu}^{-}\right)p{\pi}^{-}\right)}=0.044\pm 0.012\pm 0.007, $$

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.