glueball

A glueball is a hypothetical hadron composed entirely of gluons, the gauge bosons of quantum chromodynamics (QCD). Because gluons carry color charge, they interact with each other, making bound states of gluons possible. Glueballs would be color-singlet and electrically neutral; they can mix with ordinary mesons formed from quark-antiquark pairs, complicating identification.

Theoretical predictions: Lattice QCD calculations indicate the lightest glueball is a scalar with JPC = 0++ and a mass near 1.5 to 1.75 GeV; the next states include a tensor 2++ around 2.0–2.4 GeV and a pseudoscalar 0- around 2.6–3.0 GeV. In full QCD with dynamical quarks, mixing with qqbar states is expected, so physical resonances may be mixtures rather than pure glueballs.

Experimental status: no unambiguous glueball has been confirmed. Scalar mesons in the 1.3–1.7 GeV region, notably f0(1500) and f0(1710), have been proposed as glueball candidates, often interpreted as mixed states with considerable glueball content. Decay patterns are expected to be flavor-blind for a pure glueball but mixing and nearby qqbar states can skew decays.

Search strategies: glueball production is favored in gluon-rich processes such as radiative decays of heavy quarkonia (for example J/psi -> gamma + glueball) and central hadron production via double-Pomeron exchange. Glueballs are relatively suppressed in two-photon processes. Ongoing experimental and theoretical work aims to disentangle glueball signals from conventional mesons through decay rates, production mechanisms, and lattice-inspired mixing models.