Clear identification of glueballs and hybrid mesons

Determine which observed exotic hadron candidates predicted by Quantum ChromoDynamics (QCD) are glueballs or hybrid mesons by establishing clear, unambiguous identification criteria that distinguish these states from conventional quark–antiquark hadrons.

Background

QCD predicts hadronic structures beyond conventional quark–antiquark mesons and three-quark baryons, including glueballs (purely gluonic bound states) and hybrid mesons (quark–antiquark pairs with excited gluonic fields). Numerous experimental candidates have been reported over several decades, but disentangling them from conventional states is challenging due to mixing, overlapping mass ranges, and similar decay patterns.

Lattice QCD calculations suggest mass regions (e.g., for scalar and tensor glueballs) that overlap with established meson spectra, complicating direct assignment. While specific resonances such as f0(1500) and f0(1710) have been discussed as potential glueballs, consensus remains elusive. The review highlights that a definitive, generally accepted identification method and assignments have yet to be established.