Scientists from the University of Manchester have played a leading role in the discovery of a new subatomic particle at CERN's Large Hadron Collider (LHC). The particle, known as the Ξcc⁺ (Xi‑cc‑plus), is a new type of heavy proton-like particle containing two charm quarks and one down quark.
The result is the first particle discovery made using the upgraded LHCb detector, a major international project involving more than 1,000 scientists across 20 countries. The UK made the largest national contribution to the upgrade, with significant leadership from Manchester.
The newly observed Ξcc⁺ is a heavier relative of the proton, which was famously discovered in Manchester by Ernest Rutherford and colleagues in 1917-1919. The proton contains two up quarks and a down quark. The new discovery replaces the up quarks with their heavier relatives the charm quarks. It also extends a legacy begun in the 1950s, when Manchester physicists were the first to identify a member of the Ξ (Xi) particle family.
Professor Chris Parkes, head of the University's Department of Physics and Astronomy, led the international collaboration during the installation and first operation of the LHCb Upgrade detector. He also led the UK contribution to the project for over a decade, from approval through to delivery.
The Manchester LHCb group designed and built key components of the upgraded tracking system, the silicon pixel detector modules assembled in the University's Schuster Building. These detectors are central to precisely reconstructing the particle decays in which the Ξcc⁺ signal was observed.
Professor Parkes, said: "Rutherford's gold‑foil experiment in a Manchester basement transformed our understanding of matter, and today's discovery builds on that legacy using state‑of‑the‑art technology at CERN. Both milestones demonstrate just how far curiosity driven research can take us. This discovery showcases the extraordinary capability of the upgraded LHCb detector and the strength of UK and Manchester contributions to the experiment."
Dr Stefano De Capua, from The University of Manchester, who led the silicon detector module production, added: "The detector is a form of 'camera' that images the particles produced at the LHC and takes photographs 40 million times per second. It utilises a custom designed silicon chip that also has a variant for use in medical imaging applications."
The Ξcc⁺ particle was identified through its decay into three lighter particles (Λc⁺ K⁻ π⁺), recorded in proton‑proton collisions at the LHC in 2024, the first year of full operation of the LHCb Upgrade experiment. A clear peak of around 915 events was observed at a mass of 3619.97 MeV/c², consistent with expectations based on a previously discovered partner particle, the Ξcc⁺⁺.
This observation resolves a question that had remained open for more than two decades since an unconfirmed claim of the observation of this particle was made. The particle has now been discovered by LHCb at a mass incompatible with this earlier claim and a mass that is compatible with the theoretical expectations based on the partner particle.
In the next phase of the LHC programme, The University of Manchester is playing a leading role in LHCb Upgrade 2, which is planned to take advantage of the High-Luminosity LHC accelerator.
Professor Parkes added: "This discovery highlights the exciting scientific opportunities ahead as we prepare for the next generation of upgrades. Continued UK involvement in LHCb Upgrade 2 will be key to ensuring the UK remains at the forefront of particle physics."
Details of the Ξcc⁺ discovery are presented at the Rencontres de Moriond Electroweak conference.
