Physicists working at China's Beijing Electron Positron Collider have uncovered the strongest evidence yet that glueballs exist — exotic particles made entirely of gluons, the force-carrying particles that bind quarks together inside protons and neutrons. The results were presented last week at the International Conference on High Energy Physics (ICHEP) and posted to the preprint server arXiv.
What is a glueball?
Gluons are the particles that transmit the strong nuclear force. Under normal conditions, they exist only inside atomic nuclei, gluing quarks together. Quantum chromodynamics, the theory that describes the strong force, predicts that gluons can also bind to each other to form particles with no quarks at all. Those particles are called glueballs. For more than 50 years, physicists have searched for them without conclusive success.
The X(2370) discovery
The BES III experiment, which runs at the Beijing Electron Positron Collider II at the Institute of High Energy Physics, observed a particle called X(2370) in the decays of J/psi particles. The particle's mass matches predictions from lattice quantum chromodynamics for a pseudoscalar glueball, and its decay patterns show the flavor-singlet properties expected of a glueball-dominated state.
"It's an experimental triumph," Colin Morningstar, a particle physicist at Carnegie Mellon University who was not involved in the research, told Science. "It's the strongest evidence yet that particles dominated by a glueball component can exist in nature."
What comes next
The next step is independent verification by other experiments. Physicists point to the proposed Super Tau-Charm Facility in China and the Electron-Ion Collider under construction at Brookhaven National Laboratory in the United States as facilities that could confirm the result.
The finding, if confirmed, would show that the force particles holding matter together can themselves form matter — a prediction of quantum chromodynamics that has waited decades for experimental proof. The BES III experiment, which started in 2008, was designed in part to hunt for exactly this kind of particle.