Researchers report the first observation of the doubly charmed baryon called Ξcc+ through its decay into Λc+ K− π+. The signal is strong: the excess of events has a statistical significance above seven standard deviations. The result is based on proton–proton collision data recorded in 2024 with the upgraded LHCb Run 3 detector at a collision energy of 13.6 TeV and a total integrated luminosity of 6.9 fb−1.

The team reconstructed Λc+ particles from their decay to p K− π+ and then combined those with additional charged tracks to form candidate Ξcc+ decays. They used the detector’s precise vertexing, tracking and particle-identification systems and a full-software, real-time trigger to select events. Multivariate classifiers (boosted decision trees) were trained to separate signal from background, and simulated samples were used to model the detector response and to correct effects such as multiple scattering and photon radiation.

The measured mass of the Ξcc+ is 3619.97 ± 0.83 (statistical) ± 0.26 (systematic) +1.90/−1.30 MeV/c2. The third uncertainty, shown as an asymmetric range, reflects the effect of the unknown lifetime of the Ξcc+; the analysis assumes the lifetime lies in a specified range with a baseline value used in simulation. The difference between the masses of the Ξcc+ and the previously measured Ξcc++ is reported as −1.77 ± 0.84 ± 0.15 +1.90/−1.30 MeV/c2. The analysis applied small corrections to the measured mass to account for selection biases and final-state photon radiation.

Why this matters: the Ξcc+ is a baryon made of two charm quarks and one down quark. Finding it completes an expected isospin partner pair with the already known Ξcc++ (which has two charm quarks and an up quark). Measuring the masses and mass difference tests models of how the strong force and electromagnetic effects shape the masses of particles that contain two heavy quarks. The result also resolves, in a different production environment, a long-standing experimental puzzle: an earlier claim from the SELEX experiment gave a much lower mass for a similar state that other experiments did not confirm.