An international team working at a particle accelerator in Beijing has found the strongest evidence yet that a particle known as X(2370) is made almost entirely of gluons, a state of matter physicists first predicted nearly five decades ago.
The researchers detected clear traces of the short-lived and unstable particle, which is composed almost entirely of gluons rather than the quarks that make up ordinary matter such as protons and neutrons.
Quantum chromodynamics is the branch of the standard model of particle physics that explains how quarks and gluons combine to build ordinary matter. Quarks group together in threes to form components such as protons and neutrons, while gluons act as a kind of quantum glue holding them together. This is the same force that keeps protons and neutrons bound inside the atomic nucleus, in the way that photons carry the electromagnetic force.
For nearly 50 years, one piece of the puzzle has eluded physicists: the glueball, a rare particle predicted by theory that would be made exclusively of force carriers, with no quarks involved at all.
The role of particle X(2370)
Everything points to glueballs being the main ingredient of X(2370), a particle that the BES III Collaboration at Beijing's Institute of High Energy Physics first identified in 2011.
To hunt for it, the team uses the Beijing Electron-Positron Collider II, which smashes electrons into their antimatter counterparts at extremely high speeds. The resulting collisions produce a shower of fleeting particles, among them the J/psi meson, a particle that decays almost instantly and has become the best trail for tracking glueballs.
Reaching this point took time. Researchers recently had to analyze ten billion decay events of the J/psi meson in order to measure, for the first time, the mass and spin-parity of X(2370). The laboratory figures matched the theoretical predictions precisely.
New decay pathways confirm the finding
The decisive evidence came in the team's latest study, presented at the International Conference on High Energy Physics in Brazil and shared on arXiv, in which researchers identified new decay pathways for the particle. In simple terms, X(2370) decays into exactly the six types of quarks that exist, which further confirms its identity as a glueball.
For the scientists leading the project, confirming this new form of matter is significant. They said it puts the theory of the strong interaction to its toughest test yet and adds to what is known about the subatomic universe.
Researchers said further tests and many more collisions still lie ahead, but the alternative hypothesis to the glueball explanation has now been largely ruled out.
