'Glueball' find hailed a victory for physics

While subatomic building blocks of matter were once thought to always contain fundamental components known as quarks, Chinese scientists have confirmed the existence of a unique particle made entirely of pure force with no quarks inside.

The BESIII Collaboration, a major facility operating at the Beijing Electron Positron Collider under 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 Aug 5.

After more than a decade of data collection and over 10 billion recorded particle collisions, researchers verified that a particle designated as X (2370) is predominantly a "glueball" — a particle composed exclusively of gluons bound together.

Gluons act as subatomic "glue". They are force-carrying particles responsible for the strong nuclear force, which holds atomic nuclei together. While particles of light, known as photons, pass through one another without interacting, gluons interact directly with each other. This unique property allows them to bind together into a self-contained particle. Quantum chromodynamics, the theory describing the strong nuclear force, first predicted this "particle of pure force" in the 1970s.

"We found it 15 years ago, but confirming its identity was extremely difficult. We went through a great deal of effort and built a complete chain of evidence before announcing it to the international community," said Jin Shan, a professor at Nanjing University in Jiangsu province who led the finding.

First reported in 2011, X (2370) decays too quickly for direct measurement. Scientists instead identified it by analyzing its decay products — the secondary particles left behind.

In 2024, the team measured key quantum properties of X (2370), showing an exact match to theoretical models for the lightest type of glueball. Out of 10 billion collisions, only about 5,000 yielded the target signal.

To prove X (2370) was entirely quark-free, Jin's team looked for specific breakdown patterns that a pure glueball is forbidden from producing.

The experiment yielded zero prohibited signals, satisfying the theoretical requirements. The team also confirmed that X (2370) distributes its decay across many different particle types and rarely converts into light, further verifying its identity.

Huang Yanping, a researcher at the Institute of High Energy Physics, noted that X (2370) satisfied all theoretical predictions, providing the clearest evidence yet for glueballs and establishing a framework for future discoveries.

The search for glueballs has been one of the Beijing Electron Positron Collider's primary goals for decades. First built in the 1980s, the collider underwent a major facility upgrade in 2008 which increased data output a thousandfold, laying the groundwork for the breakthrough.

"This discovery was the result of five generations of scientists working over four decades," said Wang Yifang, an academician at the Chinese Academy of Sciences and researcher at the Institute of High Energy Physics. "Many people left along the way because the task seemed too difficult. Those who stayed finally got the answer."

The discovery has drawn praise from international experts.

Colin Morningstar, a professor at the Carnegie Mellon University in the United States, called the result "a monumental achievement for fundamental physics", while William Detmold, a theoretical physicist at the Massachusetts Institute of Technology, said the discovery ticks all the right boxes.

limenghan@chinadaily.com.cn