Observation of the Doubly Charmed Baryon Ξ_{cc}^{+} with the LHCb Run 3 Detector

The first observation of the doubly charmed baryon Ξ_{cc}^{+} is reported through its decay to the Λ_{c}^{+}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 Ξ_{cc}^{+} mass is measured to be 3619.97±0.83±0.26_{-1.30}^{+1.90}  MeV/c^{2}, where the first uncertainty is statistical, the second is systematic, and the third is due to the unknown Ξ_{cc}^{+} 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/c^{2}. This is the first observation of a new particle made with the LHCb Run 3 detector.