glueballs

Glueballs are hypothetical hadrons predicted by quantum chromodynamics (QCD). They are bound states composed entirely of gluons, the gauge bosons of the strong interaction. Because gluons carry color charge and interact with one another, QCD allows color-singlet states with no valence quarks. Glueballs would couple to ordinary hadrons and decay into meson pairs, but distinguishing them from ordinary quark–antiquark mesons is difficult in practice.

Lattice QCD calculations indicate the spectrum includes a lightest scalar glueball with J^PC = 0++ and a mass around 1.5–1.7 GeV. Additional low-lying states are predicted for 2++, 0-+, and 1-+ at higher masses near 2 GeV and above. The precise ordering and masses depend on whether dynamical quarks are included in the calculation.

Experimentally, no state is established as a pure glueball. Glueballs mix with qqbar mesons of the same quantum numbers, complicating identification. In the scalar sector, candidates such as f0(1710), f0(1500), and f0(1370) have been discussed as glueball-leaning states, but there is no consensus and interpretations vary.

Production and decay patterns offer clues. Glueballs are expected to be produced in radiative J/psi decays and in central production in hadron collisions, and to decay primarily into light meson pairs such as pi pi, K Kbar, and eta eta. Decays should be flavor-blind in the simplest picture, though mixing with qqbar affects observed rates.

Research context. Lattice QCD with dynamical quarks and other nonperturbative methods continue to refine predictions for the spectrum and mixing. Experimental searches proceed through partial-wave analyses and spectroscopy programs at various facilities, with the goal of establishing or ruling out glueball components in light mesons.