2960711 20260511095643.0 oai:cds.cern.ch:2960711 cerncds:FULLTEXT cerncds:CERN:FULLTEXT cerncds:CERN INSPIRE 3152373 CERN-EP-2026-123 arXiv arXiv:2605.04348 eng LHCB-PAPER-2026-004-003 LHCB-PAPER-2026-004 LHCb Collaboration Observation of the charmless purely baryonic decay $\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p \overline{p}}$ 2026 Geneva CERN 07 May 2026 A 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$$ \frac{\mathcal{B}(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} p\overline{p}})}{\mathcal{B}(\mathinner{\mathit{\Lambda}^0_b\!\to \mathit{\Lambda} K^+ K^-})} = (5.1 \pm 1.3_{\text{(stat)}} \pm 0.3_{\text{(syst)}}) \times 10^{-2} \,.$$ LHCb_Papers CERN PREPRINT CERN LHC LHCb LHCb Collaboration EP 2841311 741858 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.pdf Fulltext Stamped by WebSubmit: 07/05/2026 2841310 1086116 http://cds.cern.ch/record/2960711/files/LHCb-PAPER-2026-004-figures.zip Related data file(s) 2841311 135128 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.jpg?subformat=icon-1440 icon-1440 Fulltext Stamped by WebSubmit: 07/05/2026 2841311 135128 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.jpg?subformat=icon-640 icon-640 Fulltext Stamped by WebSubmit: 07/05/2026 2841311 135128 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.jpg?subformat=icon-700 icon-700 Fulltext Stamped by WebSubmit: 07/05/2026 2841311 70145 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.gif?subformat=icon icon Fulltext Stamped by WebSubmit: 07/05/2026 2841311 73094 http://cds.cern.ch/record/2960711/files/LHCb_PAPER_2026_004.jpg?subformat=icon-180 icon-180 Fulltext Stamped by WebSubmit: 07/05/2026 Preprint CC-BY-4.0 Preprint CERN 2026 SzGeCERN Particle Physics - Experiment CERN LHCb 21 May 2026 11 LHCb_Papers PREPRINT LHCBPUBDRAFTFINAL lhcb.secretariat@cern.ch n 202618