BES III finds strongest evidence yet for glueballs

Physicists at the Beijing Spectrometer III (BES III) experiment have reported the strongest evidence yet for glueballs, particles made entirely of gluons, in a preprint posted to arXiv last month and presented at the International Conference on High Energy Physics (ICHEP) last week. The findings focus on the X(2370) particle, which now matches key theoretical predictions for a glueball.

The BES III collaboration analyzed previously unreported decay modes of X(2370) and found it is a "flavor singlet," meaning it is not tied to a specific quark flavor such as up, down, or strange. The team estimates the particle is about 90 percent gluons, and its measured mass of 2.395 GeV/C² agrees with lattice QCD predictions, as do its spin and parity.

Glueballs are a direct prediction of quantum chromodynamics, the theory of the strong nuclear force, and their existence would fill a long-standing gap in the Standard Model. Most of the mass of protons and neutrons comes from the energy of gluons binding quarks together, not from the quarks themselves, so confirming glueballs would deepen understanding of how matter gets its mass.

The X(2370) particle was first discovered at BES III in 2011 with an initial mass of 2.370 GeV/C², slightly below the theoretical value. By 2024, the experiment had recorded over 10 billion J/ψ particles, allowing more precise measurements. BES III, an electron-positron collider designed to study J/ψ decays, began collecting data in 2008 and recorded over 226 million events within its first year.

Colin Morningstar, a particle physicist at Carnegie Mellon University who was not involved in the research, called the result "an experimental triumph" and "the strongest evidence yet that particles dominated by a glueball component can exist in nature," according to Science. Independent verification is the next step, possibly at the proposed Super Tau-Charm Facility in China or the Electron-Ion Collider under construction at Brookhaven National Laboratory in the US, though BES III is currently the only machine devoted exclusively to hunting for gluons.