Scientists Discover 1st Elusive ‘Glueball,’ Particle Made Of Nuclear Force

After combing data from the Beijing Spectrometer III, scientists are confident that they have found evidence for the ‘glueball’, a particle predicted by the Standard Model but which has remained elusive.

Since it went online in 2008, the Beijing Spectrometer III, an electron-positron collider, has recorded over 10 billion events forming J/ψ (Psi) particles.

Although it has shortcomings, the Standard Model has been successful in helping us understand the world around us. Over the years, the Model has provided details on the atomic structure, and we now know that nuclei contain more than protons and neutrons.

In 1974, two teams working independently, Samuel Ting’s team at Brookhaven and Burton Richter’s team at Stanford Linear Accelerator Center (SLAC), found the charm quark. It was named J (after the Chinese character, T for Ting, which looks like a J in English), while Richter chose a more conventional approach and named it ψ, following the norm in particle physics.

For years, laboratories have been researching the quark. Still, like other particles, it is fundamentally unstable and must decay and change into a species of lower energy. 

What is a glueball?

Gluons offer the nuclear force that keeps quarks together. Unlike positive or negative charges, the charge on gluons is referred to by color since they can cancel each other out, much like red, blue, and green colors cancel each other out.

Since particles like mesons consist of quarks and antiquarks, scientists believe that gluons also have anticolors such as antired, antiblue, and antigreen. Additionally, gluons can interact with themselves to create particles that are not quarks, dubbed ‘glueballs’. 

However, such particles are extremely difficult to detect in a collider experiment and are rarely discussed in scientific experiments.

Illustration explaining differences between meson, baryon and glueball. Gluons shown as springs and larger dots are quarks and antiquarks. Image credit: Brookhaven National Laboratory.

How high-performance computing helps

One reason glueballs did not get much attention is that physicists found it difficult to calculate their expected properties. With the advent of high-performance computing, techniques such as LatticeQCD have arrived.

These techniques treat spacetime as a discrete grid and, using a smaller spacing, make predictions for large-scale phenomena. 

Using this technique, physicists have estimated that the lightest glueball would not have a spin or electric charge and have a mass between 2.3 and 2.6 GeV/c². To experimentally observe a glueball, physicists at the Beijing Spectrometer III created a slightly heavier particle and watched it decay. 

When a J/ψ particle decays, there is a 26 percent chance of creating photons, nine percent chance of a decay into a photon and two gluons but a 64 percent chance of a complete decay into three gluons, a Big Think report said. 

The findings at BES III

In a recent research paper, Scientists at BES III said that they had detected a new composite particle, X (2370). The number in the brackets was the initial estimate of its mass in MeV/c2, which, after further work, has been determined to be 2395 MeV/c2 or 2.395 GeV/c², which matches mass predictions. 

More importantly, the particle has no spin and has a statistical significance of 11.7-σ. This is way above the gold standard of 5-σ, suggesting that the likelihood of the discovery was a fluke is 0.00006 percent. 

While this is not the definitive proof that scientists have spotted the glueball, it is nevertheless the strongest result to confirm that glueballs, as predicted by the Standard Model, do exist, and maybe scientists have glanced at its existence for the very first time. 

The research findings were published in the journal Physical Review Letters.