Have physicists finally discovered glueballs? New evidence points to yes.
The Beijing Spectrometer III (BES III) experiment has uncovered convincing evidence of the existence of glueballs, an elusive composite particle made entirely of gluons predicted by quantum theory. The results appeared in a preprint posted to arXiv last month and were presented at the International Conference on High Energy Physics (ICHEP) last week.
This discovery is significant because it could help answer some of the unanswered questions in particle physics, such as the existence of glueballs. According to the Standard Model of Particle Physics, glueballs should exist as a direct prediction of quantum chromodynamics, the theory of the strong nuclear force. They are also crucial in understanding why matter has mass.
As Matthew Francis wrote for Ars in 2015, glueballs are part of the reason that matter has mass. The Higgs boson, discovered in 2012, is a manifestation of the Higgs field, which is present throughout the universe. Quarks, electrons, and other fundamental particles would be mass-free in a Higgsless cosmos, but when they interact with the Higgs field, they pick up mass. In contrast, most of the mass of protons and neutrons doesn't come from quarks; it comes from the 'glue' holding them together.
Gluons, the carriers of the nuclear strong force, are the reason for that glue. Though they don't have mass, the energy involved in binding everything together inside a proton is huge, and a lot of that energy takes the form of mass thanks to E=mc2. Without gluons, protons wouldn't exist, much less be as massive as they are. But there's another side effect: gluons stick to each other, not just to quarks. That means it could be possible to build a particle out of just gluons, with no quarks needed—that's the glueball.
The J/ψ particle, discovered in 1974, is a meson consisting of one charm quark and one charm antiquark. When those particles decay, they produce a lot of gluons and composite particles known as hadrons. According to astrophysicist Ethan Siegel, for a particle to be considered a possible glueball, it must have zero spin, no electric charge, and odd parity, among other properties.
This discovery is fascinating because it could help us understand the fundamental nature of matter and the strong nuclear force. It also raises a deeper question: what other particles might exist that we haven't yet discovered? The particle zoo is vast, and glueballs are just one of many elusive particles that could be out there.
