CERN’s LHCb Collaboration Discovers the Ξ_cc⁺: A New Doubly Charmed Baryon and the Second "Heavy Proton"

On March 17, 2026, the LHCb collaboration at the European Organization for Nuclear Research (CERN) officially announced the discovery of a groundbreaking subatomic particle: the doubly charmed baryon known as Ξ_cc⁺ (Xi-cc-plus). This revelation, shared at the Rencontres de Moriond Electroweak conference, represents the first major scientific milestone following the comprehensive 2023 upgrade of the LHCb detector.

• Discovery: A new baryon within the heavy hadron family, acting as a heavy-quark analog to the proton.
• Unique Composition: It features a ccd quark structure, consisting of two charm quarks (c) and one down quark (d), making it the isospin partner of the Ξ_cc^{++} (ccu) discovered by LHCb in 2017.
• Scientific Impact: The particle allows for high-precision testing of Quantum Chromodynamics (QCD), the theory of the strong interaction that binds quarks inside atomic nuclei.
• Measured Mass: 3619.97 ± 0.83 (stat.) ± 0.26 (syst.) (+1.90 / −1.30) MeV/c², which is roughly four times the mass of a standard proton.
• Statistical Significance: The discovery reached a significance level of over 7σ, far exceeding the 5σ threshold, with the signal identified in approximately 915 events.

While a standard proton is composed of two up quarks and one down quark (uud), the Ξ_cc⁺ represents a significant "quark upgrade." In this new particle, the two light up quarks are replaced by heavy charm quarks. This unique structure means its mass is almost entirely derived from the binding energy of these heavy components, illustrating Einstein’s famous E = mc² equation in a high-energy environment.

The Ξ_cc⁺ is highly unstable and decays almost instantaneously. It follows a specific decay chain: Ξ_cc⁺ → Λ_c⁺ K⁻ π⁺, which is followed by the subsequent decay of the Λ_c⁺ into a proton, a kaon, and a pion (p K⁻ π⁺). This rapid transformation made the particle significantly harder to detect than its counterparts.

Theoretical models suggest that the lifetime of the Ξ_cc⁺ is up to six times shorter than that of its "sister" particle, the Ξ_cc^{++}, due to intricate quantum effects. This brevity explains why previous experiments, including earlier LHCb runs and the 2002 SELEX experiment, were unable to produce a conclusive signal until now.

This breakthrough was achieved using data from proton-proton collisions during Run 3 in 2024, representing an integrated luminosity of approximately 6.9 fb⁻¹. The success is attributed to the vastly improved sensitivity of the modernized LHCb detector, specifically the Upgrade I configuration, which allowed researchers to filter through the noise of billions of collisions.

Doubly charmed baryons act as an ideal laboratory for exploring QCD in the heavy-quark regime. The presence of two heavy quarks allows for more precise theoretical calculations, enabling scientists to strictly test models of strong interactions and even search for exotic states like tetraquarks and pentaquarks.

Vincenzo Vagnoni, the spokesperson for the LHCb collaboration, emphasized the importance of the find: "This is the first new particle identified since the LHCb detector upgrade in 2023, and only the second time in history that a baryon with two heavy quarks has been observed. This result will be instrumental for theorists to test the complex models of quantum chromodynamics."

Mark Thomson, the Director-General of CERN, added his perspective on the achievement: "This discovery is a brilliant example of how the unique capabilities of LHCb and our recent hardware modernizations lead directly to new scientific frontiers."

As more data is accumulated throughout Run 3, physicists intend to measure the particle's exact lifetime, spin-parity, and the probabilities of various decay channels. Looking further ahead, the search will continue for even more exotic objects, such as baryons containing three charm quarks.

The Ξ_cc⁺ marks the 80th hadron discovered by experiments at the Large Hadron Collider. While the discovery aligns perfectly with the predictions of the Standard Model, it opens a vital new chapter in our exploration of the strong nuclear force and the fundamental building blocks of the universe.

Data for this discovery was sourced from the official CERN press release, LHCb Outreach materials, and the presentation delivered at the Moriond 2026 conference.

In visual representations provided by CERN artists, the Ξ_cc⁺ is depicted as a "quark upgrade" of the proton. On the proton's family tree, it sits at the peak where both light quarks have been successfully replaced by charm quarks.