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.

References

However, exotic candidates predicted by QCD have been found but their clear identification as glueballs and hybrids is still open.

— Key Historical Experiments in Hadron Physics  (2503.14689 - Amsler, 15 Mar 2025) in Section “Conclusions”

That two careful coupled-channel analyses of the same BESIII data reach such different conclusions may itself be the most important fact about the scalar sector. The difference traces to the treatment of the broad background, of the number of poles admitted, and of the constraints imposed by other reactions. Until it is resolved, statements that the scalar glueball has been identified are premature.

— Glueballs: hadrons without quarks  (2609.16790 - Chevalier et al., 15 Sep 2026) in Section 4.1, “The scalar sector”

Against these strong expectations, the experimental situation is unresolved, for a straightforward reason: above $2$~GeV the tensor spectrum is dense.

— Glueballs: hadrons without quarks  (2609.16790 - Chevalier et al., 15 Sep 2026) in Section 4.3, “The tensor sector”

In the research of non-perturbative quantum chromodynamics (QCD), figuring out the internal structure and dynamical nature of light scalar resonances remains one of the central open problems.

— Addressing the $S$-wave scalar $f_0(1500)$-resonance in quasi-four-body FCNC rare $B_s \to f_0(1500) (\to π^+ π^- ) \ell^+ \ell^- / ν\bar ν$ decays  (2609.18890 - Zheng et al., 16 Sep 2026) in Section 1, Introduction

Whether this argument has validity, given the strong coupling between gluons and quarks, is an open question.

— Exotics with gluonic excitations  (2608.16784 - Dudek, 17 Aug 2026) in Section 1, Introduction

Identifying an observed hadron as a glueball or glueball-like state from spectroscopy remains an open question, owing in part to the possible mixing of glueball states with conventional hadrons carrying the same quantum numbers.

— Effective field theory of scalar glueballs: Form factors and interaction radii  (2610.01988 - Issifu et al., 1 Oct 2026) in Section Introduction