Observation of the Doubly Charmed Baryon Ξcc

A highly significant structure is observed in the Λþ c K−πþπþ mass spectrum, where the Λþ c baryonis reconstructed in the decay mode pK−πþ. The structure is consistent with originating from a weakly decaying particle, identified as the doubly charmed baryonΞþþ cc . The difference between the masses of the Ξþþ cc and Λþ c states is measured to be 1334.94 0.72ðstat:Þ 0.27ðsyst.Þ MeV=c2, and the Ξþþ cc mass is then determined to be 3621.40 0.72ðstat:Þ 0.27ðsyst.Þ 0.14ðΛþ c Þ MeV=c2, where the last uncertainty is due to the limited knowledge of the Λþ 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.