Particle physics has been divided for over two decades by unconfirmed claims of a particle that theory said should exist. Now, for the first time, a clean, reproducible signal has settled the question.
Scientists at CERN’s Large Hadron Collider have discovered the Ξcc⁺ (Xi-cc-plus), a subatomic particle made of two charm quarks and one down quark. According to the announcement, a clear signal of approximately 915 events was recorded at a measured mass of 3619.97 MeV/c², consistent with theoretical predictions derived from its previously discovered partner particle, the Ξcc⁺⁺.
The detection was made using the upgraded LHCb detector during proton-proton collisions in 2024, the first year the upgraded experiment ran at full capacity. Researchers identified the particle by tracking its decay into three lighter particles — Λc⁺ K⁻ π⁺ — a process captured across 40 million collision images per second.
This is the first particle discovery made using the upgraded LHCb detector.
The upgrade involved more than 1,000 researchers across 20 countries, with the United Kingdom contributing more than any other nation. The University of Manchester held a central role: its team designed and built silicon pixel detector modules assembled on-site in the university’s Schuster Building, components the announcement describes as critical for tracking particle decays with the precision needed to isolate signals like the Ξcc⁺. Professor Chris Parkes, head of Manchester’s Department of Physics and Astronomy, led the international LHCb collaboration through installation and early operation, and oversaw the UK’s involvement for more than a decade from initial approval through completion. Dr. Stefano De Capua, also from Manchester, led production of the silicon detector modules, describing the technology as “a form of ‘camera’ that images the particles produced at the LHC and takes photographs 40 million times per second” using a custom-designed silicon chip that also has applications in medical imaging.
The Ξcc⁺ belongs to the same particle family as the proton — itself first identified in Manchester by Ernest Rutherford and colleagues between 1917 and 1919. Where a proton contains two up quarks and one down quark, the newly confirmed particle replaces those up quarks with the heavier charm variety. Manchester’s connection to this particle family runs deeper still: in the 1950s, university scientists were the first to identify a member of the Xi particle family.
Professor Parkes placed the discovery in that lineage directly. “Rutherford’s gold-foil experiment in a Manchester basement transformed our understanding of matter, and today’s discovery builds on that legacy,” he said, adding that both milestones demonstrate “how far curiosity driven research can take us.”
Earlier claims of observing the Ξcc⁺ were never independently confirmed, and the mass reported in those earlier measurements does not match the figure now recorded by LHCb. The new result aligns with theoretical expectations and closes a debate that had remained open for more than 20 years.
The University of Manchester will continue its involvement with the LHCb experiment going forward, according to the announcement.
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