CERN scientists have uncovered a new proton-like particle, the Ξ_cc⁺, revealing a heavier and long-predicted member of the subatomic world.

Researchers from The University of Manchester have played a major part in identifying a previously unknown subatomic particle at CERN’s Large Hadron Collider (LHC). The particle, called the Ξ_cc⁺ (Xi-cc-plus), is a heavy proton-like particle made of two charm quarks and one down quark.

This finding marks the first particle discovery using the newly upgraded LHCb detector. The detector upgrade was a large international effort involving more than 1,000 scientists from 20 countries. The United Kingdom provided the largest national contribution to the project, with Manchester scientists taking on key leadership roles.

A Heavier Relative of the Proton

The newly detected Ξ_cc⁺ belongs to the same family as the proton, a particle discovered in Manchester by Ernest Rutherford and colleagues between 1917 and 1919. Protons are composed of two up quarks and one down quark. In the Ξ_cc⁺ particle, the two up quarks are replaced by heavier charm quarks.

This discovery also continues a long scientific tradition at Manchester. In the 1950s, physicists from the university were the first to identify a particle belonging to the Ξ (Xi) family, expanding scientists’ understanding of how quarks combine to form matter.

Manchester Leadership in the LHCb Upgrade

Professor Chris Parkes, head of the University’s Department of Physics and Astronomy, led the international collaboration during the installation and early operation of the upgraded LHCb detector. He also directed the United Kingdom’s involvement in the project for more than ten years, guiding the effort from its approval through its completion.

Scientists in Manchester designed and constructed critical parts of the detector’s tracking system. These components include silicon pixel detector modules assembled in the University’s Schuster Building. The detectors allow researchers to track particle decays with exceptional precision, making it possible to identify signals from the Ξ_cc⁺ particle.

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.”

A Detector That Captures 40 Million Particle Images Per Second

Dr. Stefano De Capua from The University of Manchester led the production of the silicon detector modules. He explained that the detector functions like an extremely fast camera.

“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.”

How Scientists Identified the Ξ_cc⁺ Particle

Researchers detected the Ξ_cc⁺ by observing how it breaks down into three lighter particles (Λc⁺ K^– π⁺). These decay products were recorded during proton-proton collisions at the LHC in 2024, the first year the upgraded LHCb experiment operated at full capacity.

Scientists observed a clear signal consisting of about 915 events with a measured mass of 3619.97 MeV/c². This measurement matches predictions based on a related particle that had previously been discovered, the Ξ_cc⁺⁺.

Resolving a Two-Decade Scientific Question

For more than twenty years, physicists debated earlier claims that this particle had been observed. Those earlier results were never confirmed. The new measurement from the LHCb experiment identifies the Ξ_cc⁺ at a mass that does not match the earlier claim but aligns with theoretical predictions based on the known partner particle.

Next Phase of CERN Research

Looking ahead, The University of Manchester is taking a leading role in the next stage of the LHC program, known as LHCb Upgrade 2. This effort will use the High-Luminosity LHC accelerator to collect far larger volumes of data and enable even more detailed studies of rare particles.