Observation of the doubly-charmed-baryon decay Ξcc++→Ξc0π+π+

A search for the doubly-charmed-baryon decay Ξcc++→Ξc0π+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 Ξcc++ baryon is observed in the Ξc0π+π+ invariant-mass spectrum. Using the Ξcc++→Λc+K−π+π+ decay as the normalisation channel, the branching fraction ratio, BΞcc++→Ξc0π+π+BΞcc++→Λc+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 as final-state interactions and resonance contributions to the hadronisation process.