Observation of the suppressed decay ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\mu }^{+}{\mu }^{-}$

The suppressed decay ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\mu }^{+}{\mu }^{-}$, excluding the $J\mathrm{/}\psi$ and $\psi (2S)\to {\mu }^{+}{\mu }^{-}$ 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 $3{\mathrm{f}\mathrm{b}}^{-1}$ collected with the LHCb experiment. The ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}{\mu }^{+}{\mu }^{-}$ branching fraction is measured relative to the $\Lambda^{0}{b}\rightarrow J/\psi(\rightarrow \mu^{+}\mu^{-})p\pi^{-}$ branching fraction giving \begin{align} \nonumber \frac{$\mathcal{B}$(\Lambda^{0}{b}\rightarrow p\pi^{-}\mu^{+}\mu^{-})}{$\mathcal{B}$({\Lambda^{0}_{b}\rightarrow J/\psi(\rightarrow \mu^{+}\mu^{-})p\pi^{-}})} &= 0.044\pm0.012\pm0.007, \end{align} where the first uncertainty is statistical and the second is systematic. This is the first observation of a $b\to d$ transition in a baryonic decay.