Physicists Discover Mysterious Glueball Particle Made Entirely of Gluons

After almost two decades of research, a team of Chinese physicists says it has found strong evidence for a long-theorized particle known as a glueball. Unlike ordinary matter, glueballs are made entirely of gluons — the force-carrying particles responsible for binding quarks together.

Researchers from the Beijing Spectrometer III (BESIII) collaboration, an international particle-physics experiment, presented their findings last week at the International Conference on High Energy Physics in Natal, Brazil. Their results suggest that X(2370), a particle first discovered in 2011, is composed largely of glueball material. Glueballs are proposed clusters of gluons, the subatomic particles that bind quarks together to form protons and neutrons inside atomic nuclei.

Bruce Yabsley, a particle physicist at the University of Sydney in Australia who reviewed the BESIII results, said the findings did not provide conclusive proof that X(2370) is a glueball. However, when considered alongside decades of research, the latest evidence is “very convincing”, he said.

“This is very convincing evidence,” said Ulrik Egede, an experimental particle physicist at Monash University in Melbourne, Australia, who attended the conference presentations.

Confirming the existence of a glueball would provide direct evidence that gluons can interact with one another — an important prediction of quantum chromodynamics, the theory describing the behaviour of quarks and gluons.

Studying glueballs could also help physicists understand the origins of mass, Yabsley said. Protons contain quarks, but the masses of those quarks account for only a small fraction of a proton’s total mass. Gluons are massless, yet the strong interactions among gluons and quarks generate most of the mass of ordinary matter.

Searching for a glueball

BESIII operates at the Beijing Electron-Positron Collider II at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences. Its location and design make it particularly well suited to the search for glueballs. Since beginning operations in 2008, the experiment has studied collisions between electrons and positrons that produce short-lived particles. Some of these particles are predicted to decay into glueballs.

Yanhping Huang, a particle physicist at IHEP, first studied X(2370) during her doctoral research.

“It was a very exciting time for us,” Huang said. The particle attracted attention because its mass was consistent with predictions for a particular type of glueball, making it one of the strongest candidates identified so far.

The particle was detected in the decay of a heavier particle known as J/ψ. According to quantum chromodynamics, J/ψ decays are an especially promising place to search for glueballs because gluons play a major role in the process, said Xiang Jing, a particle physicist at Nanjing University in China who presented the findings at the conference. BESIII has collected a large sample of J/ψ particles, allowing researchers to investigate their decays in detail.

Even so, earlier evidence was not strong enough to rule out the possibility that X(2370) was made from other types of particles, Yabsley said.

For 13 years, Huang and her colleagues in the BESIII collaboration analysed approximately 10 billion J/ψ decays. In 2024, they determined the particle’s spin parity — a quantum property that describes how it behaves under rotations and spatial reflection¹.

The results showed that X(2370) is a “pseudoscalar” particle with spin parity 0.^−＋ This matches the predicted properties of the lightest glueball.

However, the result still does not establish the particle’s identity beyond doubt. Other particles can have similar properties, Jing said, meaning that additional evidence will be needed before physicists can definitively confirm X(2370) as a glueball.