In a groundbreaking achievement for particle physics, scientists from the Chinese Academy of Sciences (CAS) have played a pivotal role in the discovery of a new exotic proton-like particle at CERN's Large Hadron Collider (LHC). This particle, dubbed Ξ_cc⁺ (Xi-cc-plus), represents a doubly charmed baryon composed of two heavy charm quarks and one lighter down quark, marking a significant milestone in understanding the strong nuclear force that binds quarks together.

The announcement, made on March 17, 2026, during the Moriond conference, highlights the upgraded capabilities of the LHCb detector and underscores the international collaboration's success, with Chinese researchers leading key aspects of the analysis. Professor He Jibo from the University of Chinese Academy of Sciences (UCAS) School of Physical Sciences spearheaded the effort, drawing on years of expertise in heavy flavor physics.

The Nature of Baryons and Quark Composition

Baryons are subatomic particles made up of three quarks, held together by the strong force mediated by gluons, as described by Quantum Chromodynamics (QCD), the fundamental theory of strong interactions. The most familiar baryon is the proton, consisting of two up quarks and one down quark (uud), with a mass of approximately 938 MeV/c². The Ξ_cc⁺ mirrors this structure but substitutes the two up quarks with heavier charm quarks (cc d), resulting in a mass of 3619.97 ± 0.83 (stat) ± 0.26 (syst) MeV/c²—roughly four times that of the proton.

Charm quarks, one of the six quark flavors (up, down, charm, strange, top, bottom), are heavier and decay more rapidly, making doubly charmed baryons elusive. Their study probes QCD in regimes where perturbative calculations falter, offering insights into quark binding dynamics at short distances.

• Proton (uud): Stable, mass ~938 MeV/c²
• Ξ_cc⁺⁺ (ccu, discovered 2017): Mass ~3621 MeV/c²
• Ξ_cc⁺ (ccd, new): Mass ~3620 MeV/c², isospin partner

LHCb Experiment and Technological Upgrades

The LHCb (LHC beauty) experiment specializes in studying matter-antimatter asymmetry and heavy flavor physics, focusing on beauty (b) and charm (c) quarks. Unlike the general-purpose ATLAS and CMS detectors, LHCb's forward geometry excels at tracking particles produced close to the beam direction.

Crucial to this discovery were 2023 upgrades to the LHCb detector, enhancing tracking precision, vertex reconstruction, and particle identification via Ring Imaging Cherenkov (RICH) detectors. These improvements allowed detection of the Ξ_cc⁺ through its decay channel Ξ_cc⁺ → Λ_c⁺ K^- π⁺, where Λ_c⁺ further decays to p K^- π⁺. From proton-proton collisions at 13.6 TeV in 2024 Run 3, researchers reconstructed 915 candidates, yielding a 7σ significance—far exceeding the 5σ discovery threshold.

Historical Context: A 20-Year Quest

The search for doubly charmed baryons dates back over two decades. Early hints from the SELEX experiment suggested lighter masses, but subsequent efforts by FOCUS, BaBar, Belle, and initial LHCb runs failed to confirm them. In 2017, LHCb observed the Ξ_cc⁺⁺ (ccu) with 5.1σ significance, validating theoretical predictions.

The Ξ_cc⁺ proved more challenging due to its shorter predicted lifetime (up to six times shorter than Ξ_cc⁺⁺), governed by quantum color dynamics favoring weak decays. Upgraded LHCb's higher luminosity and precision enabled this breakthrough, the 80th hadron discovered at the LHC.

Chinese Contributions: Prof. He Jibo and UCAS Leadership

Professor He Jibo, a leading figure in experimental particle physics, has been instrumental in LHCb's heavy flavor program. With a PhD from Tsinghua University and fellowships at CERN and LAL Orsay, he has served as convener for b-hadrons and quarkonia working groups, LHCb editorial board member, and coordinator for stripping and trigger systems.

His team at UCAS, affiliated with CAS's Institute of High Energy Physics (IHEP), contributed to data analysis, simulation, and interpretation. UCAS trains elite physicists, many joining international collaborations like LHCb, fostering China's rising prominence in high-energy physics. This discovery exemplifies UCAS's role in global science, with He Jibo's prior publications on Ξ_cc searches paving the way.

China's LHCb contingent, over 100 strong, operates key computing resources and develops GenXi_cc generator for doubly charmed baryon production simulations.

Photo by Marija Zaric on Unsplash

Implications for Quantum Chromodynamics

This find tests QCD at extreme densities, where three-quark dynamics transition to multi-quark states like pentaquarks and tetraquarks—already discovered by LHCb. Ξ_cc⁺ probes 'compactification' of charm quarks, challenging lattice QCD simulations and revealing strong force behavior near 1 fm scales.

Production rates inform heavy quarkonium suppression in quark-gluon plasma, relevant to early universe studies. Future measurements of decay widths and ratios will refine models, potentially unveiling new physics beyond the Standard Model.

Technological and Methodological Advances

The discovery relied on machine learning for particle identification, vertex fitting, and background rejection. RICH detectors, crucial for kaon-pion separation, benefited from real-time calibration led by Chinese experts like Prof. He.

• Upgraded VELO (Vertex Locator): Sub-micron precision
• Scintillating Fibre (SciFi) Tracker: Higher granularity
• AI-driven reconstruction: 10x data processing speed

China's Growing Role in Global Particle Physics

CAS and UCAS have invested heavily in high-energy physics, with IHEP hosting the Beijing Electron Positron Collider (BEPCII) and planning the Circular Electron Positron Collider (CEPC). Chinese LHCb members contribute ~10% of physics output, training PhD students who lead analyses.

This success boosts China's STEM higher education, attracting talent to programs at UCAS, Tsinghua, and Peking University. It positions China as a leader in flavor physics, complementing domestic facilities like the High Energy Photon Source (HEPS).

Future Prospects and High-Luminosity LHC

The High-Luminosity LHC (HL-LHC), starting 2029, will deliver 10x data, enabling precise Ξ_cc⁺ studies, including lifetime (~100 fs) and production cross-sections. Expected discoveries include triply charmed baryons (Ω_ccc⁺) and bottom-charmed hybrids.

For Chinese researchers, this opens doors to leadership in HL-LHCb, fostering collaborations and career growth in academia and industry.

Career Opportunities in Particle Physics

This discovery inspires aspiring physicists in China. UCAS offers PhD programs in particle physics, with opportunities at CERN via fellowships. Graduates pursue roles in accelerators, detectors, and data science, vital for AI-driven experiments.

China's expanding facilities like CEPC create demand for experts, blending theory, experiment, and computation.

Photo by Vitaly Gariev on Unsplash

Broader Impacts on Science and Society

Beyond physics, LHC technologies spin off to medicine (cancer therapy), computing (Grid tech), and materials. The international LHCb collaboration—1,500 scientists from 100 institutes—exemplifies global teamwork, with China's contributions enhancing its soft power in science.

As Prof. Vincenzo Vagnoni, LHCb spokesperson, noted: “This result will help theorists test models of quantum chromodynamics.”