More than half a century ago, quantum chromodynamics (QCD) — the theory of the strong force — predicted the existence of glueballs: particles made not of quarks, but of gluons, the very force carriers that bind quarks together. Finding one would be a spectacular confirmation of the theory. Now, after 15 years of work, the BESIII Collaboration at China's BEPCII collider says it has the strongest evidence yet.
Presenting at the International Conference on High Energy Physics in Brazil, the collaboration reported that the dominant constituent of the particle X(2370) is a pseudoscalar glueball with spin-parity quantum numbers 0⁻⁺. The X(2370) was first spotted in 2011 in the decay of J/ψ particles — and the analysis that pinned down its identity used a sample of 10 billion J/ψ decays.
Glueballs are notoriously difficult to identify. They are expected to mix with ordinary quark-based mesons of the same quantum numbers, so experimentalists must show that a glueball component dominates a candidate particle rather than merely contributing to it. According to the collaboration, the new measurements show exactly that: a pseudoscalar-glueball component must dominate the X(2370).
'Searching for glueballs has been one of the collider's primary missions' since BEPCII began operations, the team notes. The result is a milestone for a field that has chased these particles since the 1970s, when lattice QCD calculations first predicted their masses.
While the identification of the X(2370) does not close the book on glueballs — the theory predicts several varieties with different quantum numbers — it marks the first time a glueball-dominated particle has been pinned down with this level of confidence: a vindication of QCD's most exotic prediction.
