BESIII Confirms X(2370) as Glueball After 10 Billion Collider Events

Q: Why does one missing decay channel make physicists think X(2370) may be matter made only of force particles?

Because that “missing” decay is exactly the kind of channel an ordinary quark-based meson would usually have, but a glueball would not strongly prefer. The key new result is that BESIII did **not** see X(2370) decaying strongly into \(K^*(892)^0\bar K^0\), setting a very low upper limit on that branching fraction. That matters because kaon-containing final states carry a specific quark flavor, including strange quarks. If X(2370) were mainly an ordinary meson built from quarks, physicists would generally expect some flavor-favored decay pattern. Instead, this channel is strongly suppressed. A glueball, by contrast, is expected to be a **flavor-singlet** state. Since it is made of gluons, not quarks, it should not inherently favor up, down or strange quark flavors. Its decays should look comparatively flavor-symmetric, with no strong preference for one quark flavor combination. The absence of the kaon-vector channel is therefore evidence that X(2370) is not behaving like a conventional quark-antiquark meson. Physicists are not relying on that one channel alone. They are combining it with two earlier clues: X(2370) is produced in radiative \(J/\psi\) decays, a gluon-rich environment suited to making glueballs, and its measured mass and quantum numbers \(0^{-+}\) match lattice-QCD expectations for the lightest pseudoscalar glueball. The missing decay channel strengthens that broader pattern by showing the particle’s decay behavior also fits the glueball picture.

3 articles · Updated · China Daily · Aug 17

X(2370) was confirmed as a predominantly gluon-only particle after BESIII analyzed more than a decade of data and over 10 billion collisions, announcing the result at a major physics conference in Brazil.
About 5,000 events carried the target signal, and the team identified the particle through its decay products because X(2370) vanishes too quickly for direct measurement.
A 2024 measurement showed quantum properties matching predictions for the lightest glueball, while researchers found zero forbidden decay signals that would have indicated hidden quark content.
The result caps a search dating to quantum chromodynamics predictions in the 1970s and was enabled by a 2008 Beijing collider upgrade that boosted data output 1,000-fold.