Physicists in Beijing find the strongest evidence yet for glueballs

The BESIII collaboration at the Beijing Electron Positron Collider has established the decisive property of a glueball for the particle X(2370). It is a flavour singlet, meaning it is neutral towards every type of quark. Only a particle built purely from gluons behaves that way.

Glueballs have been predicted for about 50 years. They have never been observed unambiguously. They would be the only known form of matter made solely of force carriers. Detecting one would confirm quantum chromodynamics exactly where its calculations are hardest.

Gluons carry the strong nuclear force. They bind the quarks inside protons and neutrons. Unlike the photon of the electromagnetic force, gluons also act on each other. The theory therefore allows them to form bound states of their own.

A closed chain of evidence

The candidate X(2370) appeared in 2011 in the decays of J/ψ mesons. Its mass is 2.3 gigaelectronvolts. That matches lattice calculations for the lightest glueball. In 2024 the team fixed its spin parity at 0−+, again as predicted.

The test for colour neutrality was still missing. Inside a glueball the colour charges of the gluons cancel each other out. That neutrality suppresses one decay route which ordinary quark particles of this mass take easily.

The collaboration analysed roughly ten billion J/ψ decays. It found no decay into K*(892) and an anti-kaon. "We found no evidence of this decay mode," the BESIII team is quoted as saying by the science portal wissenschaft.de. The suppression, the team says, argues that X(2370) is a flavour singlet.

Mass, spin parity and colour neutrality together form a closed chain. "Other interpretations cannot conclusively explain the combination of features of X(2370)," the physicists write. A pseudoscalar glueball component must dominate the particle.

BESIII presented the result at the International Conference on High Energy Physics in Brazil. According to phys.org, it is the clearest outcome of almost 50 years of searching. The accompanying preprint is posted on the arXiv server.

How pure this glueball is remains open. The collaboration speaks of a dominant component, not of a particle made of gluons alone. An admixture of quark states is still possible. Independent peer review of the preprint is still pending.