engLHCb CollaborationObservation of the charmless purely baryonic decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}}$Particle Physics - ExperimentCERN-EP-2026-123arXiv:2605.04348LHCB-PAPER-2026-004A search for the charmless purely baryonic decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}}$ is performed using proton-proton collision data recorded by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=13\,\text{TeV}$ and corresponding to an integrated luminosity of $6.0\,\text{fb}^{-1}$. The signal decay is observed with a significance of 5.1 standard deviations. Its branching fraction is measured for the first time, relative to that of the topologically similar decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} K^+ K^-}$. Contributions from intermediate charmonium resonances decaying to the $p \overline{p}$ and $K^+ K^-$ final states are explicitly excluded with a requirement on the invariant mass of the companion hadron system, $m(h\bar{h}) < 2.85\,\text{GeV}$, where $h$ stands for a proton or a charged kaon. The relative branching fraction is found to be$$ \frac{\mathcal{B}(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p\overline{p}})}{\mathcal{B}(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} K^+ K^-})} = (5.1 \pm 1.3_{\text{(stat)}} \pm 0.3_{\text{(syst)}}) \times 10^{-2} \,.$$2026-05-07http://cds.cern.ch/record/2960711http://cds.cern.ch/record/2960711oai:cds.cern.ch:2960711