After Two Decades, BESIII Collaboration in Beijing Says It Has Found Strong Evidence of the Mysterious 'Glueball'

After almost two decades of searching, a team of physicists in China says it has observed strong evidence of a mysterious particle called a glueball — a particle made entirely of force-carrying particles. Researchers at the Beijing Spectrometer III (BESIII) Collaboration, an international particle-physics experiment, presented their results at the International Conference on High Energy Physics in Natal, Brazil, last week. They say that a particle known as X(2370), which was discovered in 2011, is mostly made up of glueballs — clusters of gluons, the elementary particles that bind quarks to form protons and neutrons and hold them inside the nucleus of an atom.

Why It Matters

There is no single smoking gun that proves that this particle is made of glueballs, says Bruce Yabsley, a particle physicist at the University of Sydney, who has reviewed the results from BESIII. But looking at the cumulative evidence built over decades makes the current findings "quite persuasive." The discovery of glueballs would provide direct evidence that gluons can interact with themselves, a key prediction of quantum chromodynamics — the theory describing quarks and gluons. Observation of glueballs can also improve physicists' understanding of the origin of mass itself: although protons are made of quarks, the sum of the masses of those quarks does not add up to the total mass of a proton. Gluons are massless, but strong interactions between them must create mass.

How BESIII Got There

BESIII, which runs at the Beijing Electron–Positron Collider II at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, is uniquely placed to observe glueballs. The experiment started in 2008 and is designed to study collisions between electrons and positrons, which can create short-lived particles predicted to decay into glueballs. X(2370) was detected when a heavier particle known as a J/ψ meson broke down — and theory has long suggested that the decay of a J/ψ particle is the golden place to search for glueballs, says Shan Jin, a particle physicist at Nanjing University who presented the results. BESIII can produce vast numbers of J/ψ particles, enabling researchers to study their decays.

The Path to Spin Parity

For 13 years, Huang and other scientists at the BESIII Collaboration pored over data from nearly ten billion J/ψ decays. Finally in 2024, they determined the particle's spin parity, a quantum number that describes how a particle behaves. That result showed that X(2370) is a "pseudoscalar" particle, with a spin parity of 0−+, consistent with predictions of the lightest glueball. But it still wasn't enough — many particles can have similar properties, says Jin. The cumulative evidence presented at Natal — combining the spin parity measurement with branching-fraction analysis and decay-channel exclusions — is what made the case persuasive.

What's Next

Three checkpoints follow. Independent confirmation from competing experiments — particularly the GlueX collaboration at Jefferson Lab and the PANDA experiment at FAIR — is the obvious next step, with both groups now re-prioritizing glueball search channels. Theoretical work to extract the gluon self-coupling constant from the BESIII data will quantify whether the result is consistent with quantum chromodynamics or hints at physics beyond the Standard Model. And a proposal to extend BESIII's run through 2030 will determine whether the collaboration can collect the ten-fold increase in J/ψ decays needed to push the evidence from "quite persuasive" to "definitive."