Chinese-Led Team Presents Strongest Evidence Yet for Elusive “Glueball” Particle

• A Chinese-led team has reported the strongest evidence yet for a glueball.

• Glueballs are particles made entirely of gluons, the carriers of the strong force.

• The work used data from Beijing’s electron-positron collider over 15 years.

• Scientists say the finding is a major experimental achievement.

• The candidate particle is known as X(2370).

A Chinese-led international research effort has been praised by physicists after presenting what scientists describe as the strongest experimental evidence so far for the existence of the “glueball”, a long-predicted particle made only of force-carrying gluons.

The findings were announced last week at the International Conference on High Energy Physics in Natal, Brazil, drawing attention across the global physics community. On Chinese social media, discussion quickly turned to whether the discovery could eventually be recognised with a Nobel Prize.

Glueballs were first predicted in the early 1970s and have remained one of the most difficult unresolved questions in modern particle physics. They are thought to be formed entirely from gluons, the massless particles responsible for binding quarks together inside protons and neutrons. In simple terms, they would be particles made from the strong force itself, without the quarks found in ordinary matter.

The latest breakthrough followed 15 years of work by researchers from 15 countries. The team analysed billions of measurements collected at an underground collider in western Beijing, gradually assembling the clearest experimental case yet for the existence of the long-sought particle.

Colin Morningstar, a theoretical physicist at Carnegie Mellon University in the United States, described the work as an “experimental triumph”. Marek Karliner, a theoretical physicist at Tel Aviv University in Israel, said glueballs are among the most solid predictions of quantum theory, but confirming them in experiments has proved extraordinarily challenging.

According to Karliner, there is a strong possibility that the Beijing team has identified the particle. He said the Beijing Electron Positron Collider was particularly well suited for the search because it collides electrons with positrons at the precise energy range needed to produce the particles used in the experiment.

Compared with larger machines such as Europe’s Large Hadron Collider, which generates many types of particles at once, the Beijing collider creates a cleaner experimental environment. That lower level of background activity allowed scientists to carry out a long series of tests required to determine whether the signal matched a glueball.

Karliner said the team in Beijing had worked through an internationally accepted checklist step by step over many years, turning the effort into a major experimental achievement.

Under the Standard Model of particle physics, atomic nuclei are made of protons and neutrons, which are themselves composed of quarks. These quarks are bound together by gluons, the carriers of the strong nuclear force. While gluons normally act as the “binding material” inside matter, theory also allows them to form particles of their own.

Karliner offered a simple comparison: imagine quarks and gluons as metal balls connected by a rubber band. If the balls are removed and the rubber band is closed into a loop, what remains resembles the idea of a glueball — a particle made only from the force-carrying component.

But detecting such a particle has been extremely difficult. Glueballs can appear in several forms and are hard to separate from ordinary particles that display similar properties.

The Beijing search began with large numbers of short-lived particles produced inside the collider. As those particles decayed, they created conditions in which glueballs could potentially form.

In 2011, researchers identified an unusual signal in the collision debris. The particle was named X(2370), with “X” indicating that its identity was unknown and “2370” referring to its mass of about 2,370 million electron volts. It soon emerged as a leading glueball candidate.

Over the following 15 years, scientists continued testing X(2370), measuring its mass, quantum numbers and, most recently, a key identifying property known as a “flavour singlet”. Taken together, the results have produced a detailed body of evidence that makes it increasingly unlikely the particle is simply another known form of matter.

Even so, researchers are not claiming that X(2370) is a completely pure glueball. Their conclusion is that the particle is dominated by glueball characteristics while also containing some ordinary quark-based matter.

The exact number of gluons involved remains uncertain. Karliner said the simplest explanation would involve three gluons, although more complex structures are also possible.

Morningstar said that confirming one of quantum theory’s last major predictions would sharpen scientists’ understanding of the universe’s basic laws. He added that such knowledge may one day open the door to technological uses that are currently impossible to predict.