Beijing experiment provides proof on new kind of matter

Physicists have established the most direct proof yet that gluons — the particles that carry the strong nuclear force — can bind together to form an entirely new kind of matter, resolving a nearly 50-year quest in particle physics.

The BESIII Collaboration, a major scientific facility operating at the Beijing Electron Positron Collider by the Institute of High Energy Physics of the Chinese Academy of Sciences, announced the results in a special plenary report at the International Conference on High Energy Physics in Natal, Brazil, on Wednesday.

The particle, called X(2370), was first observed in 2011. After 15 years of accumulating data from more than 10 billion particle collisions, the team has now built a complete chain of evidence showing that its dominant component is a glueball — a particle made entirely of gluons bound together. Unlike ordinary particles composed of quarks, a glueball is predicted by theory but has never been conclusively observed.

The idea comes from quantum chromodynamics, the theory describing the strong force that holds atomic nuclei together. Gluons are to the strong force what photons are to light — they carry the force between particles. But there is a crucial difference. Photons do not interact with each other, which is why beams of light pass through one another undisturbed. However, gluons carry the very charge they transmit, meaning they can attract and bind to one another.

This self-interacting property led theorists to predict in the 1970s that gluons should be capable of forming a self-contained particle — a ball of pure force with no quarks inside. Yet finding one in the laboratory proved far harder than writing it down in equations.

The Beijing Electron Positron Collider is among the world's leading machines for producing gluon-rich environments, making it an ideal hunting ground. Since a major upgrade in 2008, the BESIII detector has recorded more than 10 billion particle decay events, providing scientists with an unmatched dataset.

The breakthrough came in stages. In 2024, the team measured the X(2370)'s spin and parity — a kind of quantum fingerprint — and found it matched exactly what theorists had predicted for the lightest glueball of its type. Its mass, measured at roughly 2,360 million electron volts, also fell squarely within the predicted range, marking a crucial step toward establishing its true identity.

But matching a fingerprint is not the same as proving identity. The decisive test was whether X(2370) possesses a property called "flavor-singlet", which means it contains absolutely no trace of quarks. Ordinary particles carry the flavor of their constituent quarks; a glueball, made purely of gluons, should have none.

The researchers looked for a specific decay pattern that a flavor-singlet particle is forbidden to produce, and found zero signal as they expected. This result is fully consistent with the theoretical predictions on the flavor-singlet property, which is the most important property of a glueball.

This is the clearest experimental result from nearly fifty years of searches for glueballs, verifying the major theoretical prediction that gluons can bind together to form a new type of matter. This result demonstrated unique advantages of the Beijing Electron Positron Collider in studies of strong interactions, according to the institute.