For half a century, physicists have searched for a particle made of pure force. Now a team in China says it has found the strongest evidence yet that the elusive 'glueball' is real.
A particle built from force
Ordinary matter is built from quarks, which are held together by particles called gluons. Gluons carry the strong force, the interaction that binds protons and neutrons inside the atomic nucleus. Theory says gluons can also clump together on their own, forming exotic particles called glueballs.
Physicists predicted glueballs in the early 1970s as part of quantum chromodynamics, the theory that describes the strong force. But detecting one has taken decades. Glueballs are unstable and decay almost instantly, leaving only traces in particle detectors.
Fifteen years of searching
The new evidence comes from the Beijing Spectrometer III (BESIII) collaboration, an international experiment at the Beijing Electron Positron Collider. Researchers from 15 countries spent 15 years sifting through billions of measurements from the underground collider.
The team focused on a particle known as X(2370), first spotted in 2011. Their analysis shows it is made mostly of gluons. A key clue: the particle has no quark flavors, which is exactly what a true glueball should look like.
A triumph for the Standard Model
The team presented its results at the International Conference on High Energy Physics in Natal, Brazil, last week. Physicists around the world welcomed the finding. US and Israeli researchers called it a 'triumph' that confirms a central prediction of modern physics.
The result does not close the case. Physicists say more measurements are needed to fully confirm that X(2370) is a glueball, and some hope to find other glueball candidates with different masses. But the evidence is the strongest yet gathered, and it brings a half-century-old puzzle close to a resolution.
If confirmed, the glueball would complete a picture of matter that physicists have built up over decades: quarks and gluons, bound by force, building everything from atoms to stars.