Scientists using the LHCb detector at CERN report the first observation of the doubly charmed baryon Ξ_cc^+. The signal appears in the decay Ξ_cc^+ → Λ_c^+ K^- π^+ and has a statistical significance above seven standard deviations. The data come from proton–proton collisions recorded in 2024 at a center‑of‑mass energy of 13.6 TeV, with a total integrated luminosity of 6.9 fb^−1. This is the first discovery of a new particle made with the upgraded LHCb Run 3 detector.

A baryon is a particle made of three quarks. The Ξ_cc^+ contains two charm quarks and one down quark (ccd). It is the partner of the previously seen Ξ_cc^{++}, which contains two charm quarks and one up quark (ccu). Theories based on the quark model predict that these two states should have almost the same production rate and a small mass difference of a few MeV. Earlier claims of a Ξ_cc^+ sighting by the SELEX experiment at a much lower mass were not confirmed by other experiments.

To find the new particle the team reconstructed the Λ_c^+ baryon in its decay to p K^- π^+, then combined it with additional charged tracks to make Ξ_cc^+ candidates. They relied on LHCb’s upgraded detector systems: a silicon pixel vertex detector to see short flight distances, a tracking system to measure particle momentum, and ring‑imaging Cherenkov detectors to tell protons, kaons and pions apart. Event selection was refined with multivariate classifiers (boosted decision trees) trained to separate signal from background, and the well‑known Ξ_cc^{++} decay was used as a control sample to tune and check the analysis. Simulated events were produced with standard tools (Pythia8 and GenXicc2.0) to model detector effects.

The measured mass of the Ξ_cc^+ is 3619.97 MeV/c^2. The quoted uncertainties are threefold: ±0.83 MeV/c^2 statistical, ±0.26 MeV/c^2 systematic, and an asymmetric uncertainty of +1.90/−1.30 MeV/c^2 that comes from the unknown lifetime of the particle. In this analysis the lifetime was allowed to vary in a range (the paper cites an assumed range and uses a baseline value of 45 femtoseconds); different lifetime assumptions change how selection effects shift the measured mass. The mass difference between Ξ_cc^+ and the known Ξ_cc^{++} is reported as −1.77 MeV/c^2 with its own statistical, systematic and lifetime‑related uncertainties.