CERN's upgraded detector finds a new heavy cousin of the proton - with two charm quarks inside

In a Manchester basement in 1917, Ernest Rutherford's team fired alpha particles at nitrogen gas and discovered the proton - a particle made of two up quarks and a down quark. A century later, using technology Rutherford could not have imagined, scientists working at CERN's Large Hadron Collider have found a heavier version of that same basic structure, with the up quarks swapped for their more massive relatives.

Two charm quarks and a clear signal

The new particle is called Xi-cc-plus. It contains two charm quarks and one down quark, making it a baryon - the same family of particles that includes protons and neutrons. But where a proton's up quarks are among the lightest quarks in nature, charm quarks are considerably heavier, giving Xi-cc-plus a mass of 3619.97 MeV/c-squared - roughly 3.9 times the proton's mass.

The particle was identified through its decay into three lighter particles in proton-proton collision data recorded in 2024, the first year of full operation for the upgraded LHCb experiment. The signal was clear: approximately 915 events forming a distinct peak at the expected mass.

This result resolves a question that has lingered for more than two decades. In the early 2000s, a different experiment claimed to have observed Xi-cc-plus, but the claim was never confirmed. The LHCb measurement finds the particle at a mass incompatible with that earlier claim but consistent with theoretical predictions based on a partner particle, Xi-cc-plus-plus (with two charm quarks and an up quark), which LHCb had previously discovered.

Manchester's continuing particle physics legacy

The University of Manchester played a central role. Professor Chris Parkes, head of Manchester's Department of Physics and Astronomy, led the international LHCb collaboration during the detector upgrade's installation and first operation. He also led the UK contribution to the project for over a decade. Manchester's LHCb group designed and built key components of the upgraded tracking system - silicon pixel detector modules assembled in the university's Schuster Building - which were essential for reconstructing the particle decays in which Xi-cc-plus appeared.

"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," Parkes said.

The connection to Manchester extends further. In the 1950s, Manchester physicists were the first to identify a member of the Xi particle family. The new discovery continues a tradition spanning nearly 70 years.

What a detector upgrade made possible

Xi-cc-plus is the first particle discovery from 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.

Dr. Stefano De Capua from Manchester, who led silicon detector module production, described the technology: the detector is a form of camera that images particles produced at the LHC, taking photographs 40 million times per second using a custom-designed silicon chip that also has a variant for medical imaging applications.

The upgraded detector's improved resolution and data throughput were critical for extracting the Xi-cc-plus signal from the enormous background of other particle interactions in LHC collision data.

What doubly-charmed baryons teach us

Particles containing two heavy quarks are interesting because they test our understanding of the strong force - the fundamental interaction that binds quarks together inside protons, neutrons, and all other hadrons. The strong force, described by quantum chromodynamics (QCD), is notoriously difficult to calculate in the regime where quarks are bound inside particles. Doubly-charmed baryons provide a test case where theoretical predictions can be compared against experimental measurements with unusual precision.

The mass measurement, the decay properties, and the production rates of Xi-cc-plus all constrain QCD models in ways that singly-charmed or light-quark particles cannot. Each new doubly-heavy baryon discovered adds another data point to a picture that theorists have been trying to complete for decades.

The discovery was presented at the Rencontres de Moriond Electroweak conference. In the next phase of the LHC programme, Manchester is playing a leading role in LHCb Upgrade 2, planned to take advantage of the High-Luminosity LHC accelerator - which will produce even more collisions and potentially reveal rarer particles still.