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

The first observation of a charmless purely baryonic decay, ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }p\bar{p}$, is reported using the full Run 2 LHCb dataset, corresponding to an integrated luminosity of $6.0f{b}^{-1}$. The branching fraction is measured relative to that of the topologically similar normalisation mode ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{K}^{+}{K}^{-}$. 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 $(5.13\pm {1.28}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.27}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}})\times {10}^{-2}$ in the region $m(h^{+}{h}^{-})<2.85$ GeV.