Observation of the doubly charmed baryon ${\Xi }_{cc}^{+}$ with the LHCb Run 3 detector

The first observation of the doubly charmed baryon ${\Xi }_{cc}^{+}$ is reported through its decay to the ${\Lambda }_c^{+}{K}^{-}{\pi }^{+}$ 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 $6.9{\mathrm{f}\mathrm{b}}^{-1}$. The ${\Xi }_{cc}^{+}$ mass is measured to be $3619.97\pm 0.83\pm 0.26{}_{-1.30}^{+1.90}\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, where the first uncertainty is statistical, the second is systematic, and the third is due to the unknown ${\Xi }_{cc}^{+}$ 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 ${\Xi }_{cc}^{+}$ and ${\Xi }_{cc}^{++}$ baryons is determined to be $-1.77\pm 0.84\pm 0.15{}_{-1.30}^{+1.90}\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$. This is the first observation of a new particle made with the LHCb Run 3 detector.