Physicists discover a heavy cousin of the proton at CERN’s Large Hadron Collider

This marks the first particle discovery using the upgraded LHCb detector. The upgrade is part of a major international effort involving more than 1,000 researchers across 20 countries. The United Kingdom contributed more than any other nation, with Manchester providing significant leadership.

A Heavier Relative of the Proton

The newly discovered Ξ_cc⁺ is part of the same family as the proton, which was first identified in Manchester by Ernest Rutherford and colleagues between 1917-1919. While a proton contains two up quarks and one down quark, the Ξ_cc⁺ replaces the up quarks with heavier charm quarks.

This finding also builds on a long history of particle physics research at Manchester. In the 1950s, scientists at the university were the first to identify a member of the Ξ (Xi) particle family, laying the groundwork for discoveries like this one.

Manchester’s Role in the LHCb Detector 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 oversaw the United Kingdom’s involvement in the project for more than ten years, guiding it from initial approval through completion.

The Manchester LHCb team designed and built essential parts of the upgraded tracking system, including silicon pixel detector modules assembled in the University’s Schuster Building. These components are critical for accurately tracking particle decays and identifying signals such as the Ξ_cc⁺.

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.”

Advanced Detector Captures Particle Collisions

Dr. Stefano De Capua from The University of Manchester led the production of the silicon detector modules. He described the detector as operating like a high-speed 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 the Ξ_cc⁺ Particle Was Identified

Researchers detected the Ξ_cc⁺ by observing how it decays into three lighter particles (Λ_c⁺ K⁻ π⁺). These decay events were recorded during proton-proton collisions at the LHC in 2024, the first year the upgraded LHCb experiment ran at full capacity.

A clear signal of about 915 events was measured at a mass of 3619.97 MeV/c². This result matches predictions based on a previously discovered related particle, the Ξ_cc⁺⁺.

Solving a Two-Decade Mystery in Particle Physics

For more than 20 years, scientists had debated earlier claims that this particle had been observed, but those findings were never confirmed. The new measurement from LHCb places the particle at a mass that does not match the earlier claim but does agree with theoretical expectations based on its partner particle.

What Comes Next for CERN and Manchester

Looking ahead, The University of Manchester will continue to play a leading role in the next phase of the LHC program, known as LHCb Upgrade 2. This upgrade will take advantage of the High-Luminosity LHC accelerator to gather more data and explore rare particles in greater detail.

Details of the Ξ_cc⁺ discovery are presented at the Rencontres de Moriond Electroweak conference.