CERN's LHCb detector discovers new Ξcc⁺ particle with two charm quarks and a down quark

A team of physicists from CERN and the University of Manchester, led by Patrick Spradlin, reported on May 26, 2026 in Physical Review Letters the discovery of a new subatomic particle labelled Ξcc⁺.

The particle belongs to the family of heavy baryons. It contains two charm quarks and one down quark, making it roughly four times heavier than a proton. The distinctive aspect of the discovery is that this is the first new particle detected after the LHCb detector upgrade was completed in 2024, confirming that the upgraded equipment is meeting its design sensitivity.

What is new in the heavy-baryon family

The discovery follows more than two decades of systematic searches for particles with two charm quarks. These particles were predicted in the 1960s within the Standard Model of particle physics.

The first member of the same family, Ξcc⁺⁺, containing two charm quarks and an up quark, was discovered in 2017, also at LHCb. Finding a new member of the family matters because it enables:

• direct comparison of masses across related baryons
• measurement of decay lifetimes
• analysis of branching ratios across decay channels

Each of these is a precision test of the Standard Model itself.

Methodology

The experiment was carried out during the Run 3 proton-proton collision campaign at the LHC, at a centre-of-mass energy of 13.6 teraelectronvolts. The team analysed more than 8 billion collisions, looking for the characteristic decay signature of Ξcc⁺ into a Λc⁺ baryon plus a pion.

The final signal was extracted with a statistical significance of 5.4 sigma, above the threshold for officially declaring a discovery. The measured mass of the new particle is 3,518.1 megaelectronvolts, with an uncertainty of about 0.5 megaelectronvolts. The lifetime is approximately 0.11 picoseconds.

The upgraded LHCb detector can now process collisions five times faster per second than its previous configuration. That capability is decisive for detecting rare decay channels. Spradlin’s team had to use new machine learning algorithms to extract the signal from background noise, particularly to distinguish Ξcc⁺ decays from similar but known decays of other baryons. The algorithms were trained on simulated data and validated on a control sample of known particles.

What the discovery does NOT confirm

Although it is confirmed at high statistical significance, the authors note this is not a result from so-called physics beyond the Standard Model. The new particle was predicted by existing theory, and its detection is confirmation rather than a turning point.

More precise measurements of the mass and decay channels of Ξcc⁺ will only now make possible a detailed check of quantum chromodynamics in the heavy-quark regime, where theoretical predictions carry large uncertainty.

The discovery of Ξcc⁺ does not answer the questions that remain open in particle physics:

• the nature of dark matter
• the asymmetry between matter and antimatter
• neutrino masses

All of this remains outside the reach of current LHC experiments. It should also not be confused with the searches for rare B-meson decays, which in recent years have shown deviations from the Standard Model at about the 4 sigma level. Those searches look for hints of new physics, whereas Ξcc⁺ is more confirmation than surprise.

Next steps

Spradlin’s team plans to measure the masses of other predicted baryons in the same family, particularly particles containing charm and strange quarks. The team is also working on analysing rare Ξcc⁺ decay channels that could show deviations from theoretical predictions.

Similar work is being done in parallel at the Belle II experiment in Japan, giving an independent confirmation route. Serbia participates in both experiments through physicists at the Vinca Institute, who contribute to data analysis and the development of simulation tools.

Will CERN nevertheless detect the first hints of physics beyond the Standard Model by the end of the decade?

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