Observation of the doubly-charmed-baryon decay Ξ⁺⁺_cc → Ξ⁰_cπ⁺π⁺

Abstract: A search for the doubly-charmed-baryon decay Ξ⁺⁺_cc → Ξ⁰_cπ⁺π⁺ is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 5.4 fb⁻¹. A significant structure consistent with the Ξ⁺⁺_cc baryon is observed in the Ξ⁰_cπ⁺π⁺ invariant-mass spectrum. Using the Ξ++cc → Λ⁺_c K⁻π⁺π⁺ decay as the normalisation channel, the branching fraction ratio, B(Ξ⁺⁺_cc → Ξ⁰_cπ⁺π⁺) / B(Ξ⁺⁺_cc → Λ⁺_c K⁻π⁺π⁺), is measured to be 1.37 ± 0.18 (stat) ± 0.09 (syst) ± 0.35 (ext). This measurement provides critical input for testing QCD factorisation methods in the weak decays of doubly-heavy baryons, particularly in quantifying nonperturbative effects such asfinal-state interactions and resonance contributions to the hadronisation process.