New evidence from China’s BESIII experiment suggests the long-mysterious X(2370) may be the elusive “glueball” predicted by quantum chromodynamics, offering a rare glimpse into how the strong nuclear force builds matter

After decades of searching, physicists may finally have found compelling evidence for one of particle physics’ most unusual predictions: a particle made not from ordinary matter, but predominantly from the carriers of the strong nuclear force itself.

Researchers working with the Beijing Spectrometer III, or BESIII, experiment in China say that a particle known as X(2370) appears to be largely composed of gluons — the elementary particles responsible for binding quarks together inside protons and neutrons. Such a state is known as a glueball, and its existence has long been predicted by quantum chromodynamics, the theory that describes the strong interaction.

The findings were presented this month at the International Conference on High Energy Physics in Natal, Brazil, following roughly 15 years of work by the BESIII collaboration. Researchers say the accumulated evidence now points strongly toward X(2370) being dominated by a pseudoscalar glueball component.

If confirmed, the result would provide an important experimental test of one of the stranger properties of the strong force.

Gluons normally act as the “glue” holding quarks together. Three quarks combine to form particles such as protons and neutrons, while quark-antiquark pairs form another family of particles known as mesons.

But gluons are different from the particles carrying some other fundamental forces. Because gluons themselves carry the type of charge associated with the strong interaction, they can interact directly with one another.

The mathematics of quantum chromodynamics therefore predicts that gluons should sometimes bind together without needing quarks at all, forming standalone composite particles: glueballs.

Physicists have searched for convincing examples for decades.

The difficulty is that glueballs are expected to mix with ordinary particles made of quarks, making them extremely difficult to identify experimentally. Many candidate particles have appeared over the years, but none has provided universally accepted evidence.

X(2370) has been one of the most promising.

The particle was first observed by BESIII researchers in 2011. Its mass immediately attracted attention because it was broadly compatible with theoretical calculations for a particular type of glueball.

But mass alone was not enough.

Over the following years, researchers accumulated increasingly detailed measurements of the particle’s properties. A particularly important result came in 2024, when BESIII determined its spin and parity — quantum characteristics that describe how a particle behaves.

X(2370) was found to have spin-parity quantum numbers of 0⁻⁺, matching predictions for the lightest pseudoscalar glueball.

Even that was not definitive. Other particles containing quarks can possess similar quantum properties.

The latest analysis goes further.

Using an enormous dataset containing nearly 10 billion decays of J/ψ particles, researchers examined how X(2370) is produced and how it subsequently decays. J/ψ decays are considered particularly promising environments for producing glueballs because they can generate large concentrations of gluons.

According to BESIII, the pattern emerging from these measurements is best explained if the X(2370) contains a dominant glueball component. The collaboration now describes it as a glueball-dominated particle rather than merely a possible candidate.

Independent physicists have reacted cautiously but positively.

Bruce Yabsley, a particle physicist at the University of Sydney, told Nature that there was no single decisive measurement proving the case, but said that the cumulative evidence was “quite persuasive”. Ulrik Egede of Monash University similarly described the evidence as convincing.

That caution is important.

Particle physics has a long history of intriguing signals disappearing when more data become available. Researchers will want independent analyses, improved theoretical calculations and additional measurements before declaring the decades-long search definitively over.

Nevertheless, the potential importance of the discovery extends well beyond adding another particle to the subatomic catalogue.

Glueballs could help physicists better understand one of the most fundamental puzzles in modern physics: where the mass of ordinary matter actually comes from.

Protons and neutrons account for nearly all the mass of visible matter around us, yet the quarks inside them contribute only a relatively small fraction of that mass directly.

Most of a proton’s mass emerges instead from the extraordinarily energetic interactions among its quarks and gluons.

Gluons themselves are massless, but the energy stored in their interactions contributes to the mass of composite particles through Einstein’s relationship between mass and energy.

Studying a particle composed primarily of interacting gluons could therefore give scientists an unusually clean laboratory for investigating how the strong force effectively generates mass.

The discovery would also represent a significant confirmation of quantum chromodynamics.

QCD has been enormously successful in explaining the behaviour of quarks and gluons, but the theory becomes exceptionally difficult to calculate in situations where the strong force becomes truly strong.

Physicists often rely on enormous computer simulations known as lattice QCD to predict the properties of particles such as glueballs. Finding those states experimentally would give researchers a powerful way to test whether those calculations accurately describe nature.

The BESIII result is particularly notable because glueballs would represent an exotic form of matter unlike familiar atoms.

Ordinary objects are ultimately assembled from electrons and nuclei containing protons and neutrons. Those particles, in turn, contain quarks.

A genuine glueball would occupy a very different category: a composite particle formed predominantly from the carriers of a fundamental force binding themselves together.

That makes the idea conceptually striking. The particle would not simply be another unusual arrangement of matter. It would be, in a meaningful sense, a particle built largely from force itself.

Researchers are now expected to continue studying X(2370) and search for additional glueball states predicted by theory. Finding an entire family of such particles, with masses and quantum properties matching QCD calculations, would make the interpretation considerably stronger.

For the moment, physicists are stopping short of calling the mystery completely solved.

But after decades in which glueballs existed primarily inside equations, computer simulations and lists of hypothetical particles, the evidence has moved significantly closer to the laboratory.

If X(2370) ultimately survives that scrutiny, it could become one of the clearest demonstrations yet of how the invisible forces operating inside every atomic nucleus create the matter that makes up the visible universe.