The LHCb Collaboration has found a new particle consisting of one strange quark and two charm quarks. With this finding, the Collaboration has discovered the final member of a family of doubly charmed baryons – particles made up of two charm quarks and one other quark – and closes a chapter of a story that stretches back more than sixty years. 

Quarks are basic building blocks of matter. There are six types of quarks (up, down, charm, strange, top and bottom), which bond into pairs or triplets, known as mesons and baryons respectively. Sixty years ago, as experiments started to reveal the underlying quark structure of matter, researchers began to build theoretical models to classify how quarks can combine into composite particles. Soon, scientists were able to predict the properties of as-yet-undiscovered particles.

In 1964, the discovery of a new particle at Brookhaven National Laboratory marked a turning point. Consisting of three strange quarks, this particle had already been predicted by theorists, and the experimental confirmation of its existence and properties showed the strength of these theoretical models.

Ten years later, in 1974, another discovery rocked the world of particle physics as it revealed a fourth quark, the charm quark. This meant that theorists now had to extend their models to accommodate the many new possible quark combinations. Included in these predictions were the doubly charmed baryons. These are particles that each consist of two charm quarks and either an up, a down or a strange quark as the third of the triplet. Physicists are particularly interested in this family of particles as the large mass differences between the quarks could provide useful insight into the strong force, which binds quarks together into composite particles.

However, the experiments of the time were neither able to produce the doubly charmed baryons, nor did they have sensitive enough equipment to detect them. For comparison, the landmark discovery of the particle consisting of three strange quarks in 1964 was made by searching through 80 000 photographs of particle collisions in bubble chambers. It would take the far higher energies of the Large Hadron Collider (LHC) and the combing through of trillions of particle collisions for researchers to begin to search for the doubly charged baryons in earnest.

Comparison of the changing techniques of particle detection. Left: a photograph of the 1964 discovery of a particle consisting of three strange quarks using a bubble chamber (Image: Brookhaven National Laboratory). Right: a reconstruction of how the new particle of two charm quarks and one strange quark was created and detected at the LHCb experiment (Image: CERN).

The LHCb Collaboration discovered the first of these doubly charmed baryons in 2017 and the second earlier this year. Now, 50 years after they were first predicted, the third and final member of this family of doubly charmed baryons has been found.

This discovery is based on data collected in 2024 from high-energy proton–proton collisions at the LHC. These collisions produced the new doubly charmed baryons, which are short-lived, travelling a fraction of a millimetre in the detector before decaying into more stable particles. The LHCb Collaboration traced the tracks left by these particles in the detector back to their points of origin. This revealed the characteristic signature of the new short-lived particle with a distinct mass around four times heavier than a proton.

“This is a moment of beautiful historical significance,” said Paula Collins, incoming Deputy Spokesperson of the LHCb experiment. “Out of the 85 composite particles discovered so far at the LHC, these three doubly charmed baryons are unique. They decay by the weak force and live long enough to give measurable flight distances in our experiment. The discovery was made possible thanks to the LHCb’s upgraded detector, with its powerful capabilities to track and identify particles.”

But the story is not over yet. Future research will use the latest data from the LHCb experiment to make precision measurements of the properties of this new particle, including its mass and lifetime, and to refine those of the other doubly charmed baryons. In addition, excited counterparts of these particles remain to be observed, as do new baryons containing beauty quarks, either in place of or in addition to charm quarks.

As the LHC now transforms into the High-Luminosity LHC, the LHCb experiment is planning major upgrades to its detectors in the 2030s. Researchers hope that this will bring many of these undetected particles within reach, allowing the LHCb to start a new chapter of particle discovery.