Observation of the Doubly Charmed Baryon Ξ[subscript cc]⁺⁺

Abstract

A highly significant structure is observed in the Λ[subscript c]⁺K⁻π⁺π⁺ mass spectrum, where the Λ[subscript c]⁺ baryon is reconstructed in the decay mode pK-π⁺. The structure is consistent with originating from a weakly decaying particle, identified as the doubly charmed baryon Ξ[subscript cc]⁺⁺. The difference between the masses of the Ξ[subscript cc]⁺⁺ and Λc⁺ states is measured to be 1334.94±0.72(stat.)±0.27(syst.) MeV/c², and the Ξ[subscript cc]⁺⁺ mass is then determined to be 3621.40±0.72(stat.)±0.27(syst.)±0.14(Λ[subscript c]⁺) MeV/c², where the last uncertainty is due to the limited knowledge of the Λ[subscript c]⁺ mass. The state is observed in a sample of proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.7 fb⁻¹, and confirmed in an additional sample of data collected at 8 TeV.