Physicists Have Been Chasing a Mysterious Particle for 50 Years—and They May Have Just Found It

3 min readHere’s what you’ll learn when you read this story:For decades, scientists have searched for the “glueball”—a hypothetical particle made mostly of gluons, and the only force-carrying particle with a charge. In 2011, the Beijing Electron Positron Collider found a compelling candidate known as X(2370), and in recent years, that claim has only strengthened. Now, new research shows that X(2370) behaves as a flavor-singlet, meaning it binds to any flavor of quark—a strong indicator that the particle is the glueball.For more than two hundred years, particle physicists have been—to borrow an allusion from Alice in Wonderland—exploring just how deep the atomic rabbit hole goes. A big leap forward came at the beginning of the 19th century, when scientist John Dalton began investigating modern atomic theory. Fast-forward to the end of that century, and physicist J.J. Thomson discovered the all-important electron, and by the end of the 1960s, physicists at the Stanford Linear Accelerator Center (SLAC) had discovered the existence of quarks—subatomic particles that form the constituent parts of protons and neutrons.As far as we know, quarks are the fundamental building blocks of all matter. But for decades, scientists have still been sorting out all the particles that can result from this subatomic soup, and they have become hung up on one particularly elusive particle: the “glueball,” which is made almost entirely of gluons. Gluons are the force-carrying particles of the strong nuclear force, which is one of the four fundamental forces of the universe, along with the weak nuclear force, electromagnetism, and gravity. And as their name suggests, gluons are thought to act as the glue that binds protons and neutrons together in the atomic nucleus.As Science notes, other force-carrying particles—photons, w and z bosons, and gravitons (maybe)—don’t interact with each other. But gluons possess a charge, meaning that they could theoretically bind together, according to quantum chromodynamics (QCD). Those bound-together particles, made of mostly gluons, are what scientists call “glueballs.”In 2011, a scientific team from the BESIII Collaboration at the Beijing Electron Positron Collider, also known as BESIII Collaboration, detected a possible glueball. Dubbed X(2370), the particle matched some of the properties predicted by QCD, but the results remained uncertain. But the team made a major breakthrough in 2024, when they determined that the particle’s mass and quantum identity also matched predictions. The key to the team’s success is that the collider produces a specific meson, J/ψ, whose decay products are rich in gluons. Of course, all of this happens near-simultaneously, and sorting through the results requires complicated mathematics.“The radiative decay of the J/ψ meson is a gluon-rich process and is therefore regarded as an ideal place for searching and studying glueballs,” the team wrote in a 2024 study in the journal Physical Review Letters. Two years later, in a preprint published on server arXiv (and delivered as a presentation at this month’s International Conference on High Energy Physics in Brazil), the BESIII Collaboration provided even more evidence that particle X(2370) is indeed the glueball they’ve been looking for. By looking at 10 billion J/Ψ decay events, the team discovered that X(2370) is a “flavor singlet,” meaning the particle doesn’t favor any one of the six “flavors” of quarks—up, down, strange, charm, top, or bottom. Gluons are thought to interact equally with all flavors of quarks, and the researchers say that this flavor-singlet quality is “the most important characteristic of a glueball.”Verifying the results independently could take years, as the collider in Beijing is the only one dedicated to hunting for glueballs. But for now, it appears that yet another QCD prediction has been proven true.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.