Observation of the charmless purely baryonic decay $\Lambda_b^{0} \to \Lambda p \bar{p}$ at LHCb

The first observation of a charmless purely baryonic decay, $\Lambda_b^{0} \to \Lambda p \bar{p}$, is reported using the full Run 2 LHCb dataset, corresponding to an integrated luminosity of $6.0~fb^{-1}$. The branching fraction is measured relative to that of the topologically similar normalisation mode $\Lambda_b^{0} \to \Lambda K^+K^-$. A simultaneous fit to the long- and downstream-track categories yields a signal significance of $5.1\sigma$ after including systematic uncertainties. The relative branching fraction is measured to be $\left(5.13 \pm 1.28_{\rm stat} \pm 0.27_{\rm syst}\right)\times 10^{-2}$ in the region $m(h^+h^-)< 2.85$ GeV.