Observation of the Doubly Charmed Baryon Ξcc+ with the LHCb Run 3 Detector

The first observation of the doubly charmed baryon Ξcc+ is reported through its decay to the Λc+K-π+ final state, with a statistical significance exceeding seven standard deviations. The observation is made using proton-proton collision data collected in 2024 with the LHCb Run 3 detector at a center-of-mass energy of 13.6 TeV, corresponding to a total integrated luminosity of 6.9 fb^-1. The Ξcc+ mass is measured to be 3619.97±0.83±0.26_-1.30^+1.90 MeV/c², where the first uncertainty is statistical, the second is systematic, and the third is due to the unknown Ξcc+ lifetime, which is assumed to lie in the range 15-160 fs with a baseline value of 45 fs. The difference between the masses of the Ξ_cc⁺ and Ξ_cc⁺⁺ baryons is determined to be -1.77±0.84±0.15_-1.30^+1.90 MeV/c². This is the first observation of a new particle made with the LHCb Run 3 detector.