Scientists May Have Finally Observed A Glueball

One of the first particles predicted by quantum chromodynamics (QCD), which consists of the laws of physics related to the strong force, whose properties have been calculated for almost fifty years, is the glueball. The glueball is a strong force bound composite particle made up of gluons, but without any valance quarks. 

Its properties were fairly easy to predict early on, because they are, at tree-level, a function of only experimentally determined Standard Model parameter, the strong force coupling constant (which is about 0.1180(9) at the Z boson mass energy scale), and because there are significant symmetries involved in glueball structure in the Standard Model.

They are hard to detect, in part, because glueballs are bosons and can blend into resonances of non-glueball particles with the same quantum numbers.

In the Standard Model (SM) of particle physics, gluons are the fundamental particles mediating the strong interaction, as photons do in electromagnetic interactions. Gluons can attract each other to form new bound states called glueballs, which are the only particles in nature entirely composed of force mediators. Finding these gluon bound states is crucial and serves as a fundamental test of the SM. No candidate has yet been unambiguously identified until the new BESIII result to be reported in this presentation. 

The Beijing Electron Positron Collider (BEPCII) is a double ring e+e- collider in the 2-5 GeV energy region and the Beijing Spectrometer (BESIII) is a general purpose detector operating at the BEPCII. Decays of J/ψ particle produce a gluon-rich environment and are an ideal place to search for glueball. Discovery of the glueball has been one of the most important scientific goals of the BEPC and BEPCII for decades. 

The X(2370) particle was first discovered at the BESIII experiments in 2011. To confirm its pseudoscalar glueball state nature experimentally, the most crucial step is to determine whether the spin parity quantum number of the X(2370) are indeed zero spin and negative parity. 

Recently, based on ~10 billion J/ψ decays collected with BESIII, the spin-parity quantum numbers of the X(2370) were firstly measured with a complex partial wave analysis. The experimental results, including quantum numbers, mass, production and decay properties, are consistent with the features of the lightest pseudoscalar glueball. This recent study provides direct and strong experimental evidence for X(2370) being a glueball.

From a CERN Press Release (May 11, 2024).

Wikipedia notes that:

In 2024, the X(2370) particle was determined to have mass and spin parity consistent with that of a glueball. However, other exotic particle candidates such as a tetraquark could not be ruled out.

In support of those conclusions Wikipedia cites the following two papers:

1.  Ablikim, M.; et al. (BESIII Collaboration) (May 2024). "Determination of Spin-Parity Quantum Numbers of X(2370) as 0⁻⁺ from J/ψ → γK0
   SK0
   η′". 132 (18) Physical Review Letters 181901. arXiv:2312.05324 doi:10.1103/PhysRevLett.132.181901; and
2. "New particle at last! Physicists detect the first "glueball". Big Think. 2024-05-07.

Lattice QCD predictions have predicted the existence a pseudoscalar glueball with the following properties:

0−+	2590±130 MeV/c2

At the low end of the two sigma range, this would be a mass of 2330 MeV, so a pseudoscalar particle with a mass of 2370 MeV would be consistent with this prediction. The measured mass of 2395 MeV is even closer to the predicted mass (about 1.5 sigma).

More recent calculations of the lightest pseudo-scalar glueball properties that those cited in Wikipedia, however, are even closer according to the Big Think article cited above: "the latest theoretical results from Lattice QCD, published in 2019, predicted a mass of 2.395 ± 0.014 GeV/c²," which is a perfect match to the experimental result.

Until now, it had remained an open possibility that glueballs could not exist without valence quark sources, even though the Standard Model contained no rule requiring that rule. A new Standard Model rule imposing that requirement, however, would have been fairly easy to add to the Standard Model if observation had required it, and wouldn't have added any experimentally observed parameters to the Standard Model.

In additional to finally realizing a Standard Model prediction that is half a century old, the mass of this particle also provides a particularly clean way to reverse engineer the strong force coupling constant.