Definition
A hexaquark is a hypothetical composite particle in quantum chromodynamics (QCD) consisting of six quarks bound together by the strong nuclear force. It is a type of exotic hadron and is often classified as a dibaryon, meaning it can be viewed as a bound state of two conventional baryons (each containing three quarks).
Theoretical Background
In the standard model of particle physics, quarks combine in color‑neutral configurations. The most familiar arrangements are mesons (quark–antiquark) and baryons (three quarks). QCD does not forbid higher‑order color‑singlet combinations, and the six‑quark configuration is permitted as a color‑neutral state. Calculations using lattice QCD, quark‑cluster models, and the MIT bag model have explored the possible binding mechanisms, suggesting that certain flavor‑symmetric combinations could be energetically favorable.
Notable Proposed States
| Proposed state | Quark content | Predicted properties | Experimental status |
|---|---|---|---|
| H‑dibaryon | u d s u d s (two up, two down, two strange) | Predicted mass ≈ 2.2 GeV/c²; possible stability against strong decay | No conclusive observation; searches in hypernuclear decays and heavy‑ion collisions have yielded only upper limits |
| d*(2380) | u u d d u d (six light quarks) | Observed resonance at 2.38 GeV/c² with width ≈ 70 MeV in double‑pion production; interpreted by some as a hexaquark‑type dibaryon | Recognized as a dibaryon resonance; its internal structure (compact hexaquark vs. molecular) remains debated |
| Other flavor‑symmetric candidates | Various combinations (e.g., u u u d d d) | Theoretical calculations predict possible bound or resonant states near the baryon‑baryon thresholds | No experimentally confirmed examples aside from d*(2380) |
Experimental Searches
Searches for hexaquarks have employed a range of techniques:
- Hypernuclear decay experiments – looking for anomalous decay signatures that could indicate an H‑dibaryon escaping detection.
- Heavy‑ion collisions – analyzing invariant mass spectra for narrow structures in the baryon‑baryon channel.
- Photon‑induced reactions – probing two‑nucleon systems at facilities such as Jefferson Lab and MAMI.
To date, no hexaquark has been unequivocally observed as a stable particle. The d*(2380) resonance is the most robust candidate, but its classification as a compact six‑quark state versus a loosely bound baryon‑baryon molecule is not settled.
Current Status
The existence of bound hexaquarks remains an open question in hadron physics. While theoretical frameworks allow such states, empirical evidence is limited to resonant structures whose internal composition is not definitively determined. Ongoing experiments at high‑intensity facilities (e.g., J‑PARC, FAIR) aim to improve sensitivity to possible hexaquark signals.
Related Concepts
- Dibaryon – a broader category encompassing any six‑quark state, including molecular baryon‑baryon bound states.
- Exotic hadron – hadrons whose quark content exceeds the conventional meson or baryon configurations (e.g., tetraquarks, pentaquarks).
- Lattice QCD – a non‑perturbative computational technique used to study the binding energies of multi‑quark systems.
References
- Jaffe, R. L. (1977). “Perhaps a Stable Dihyperon.” Physical Review Letters, 38(4), 195–198.
- Bashkanov, M., et al. (2009). “Observation of a New Resonance in Double‑Pion Production.” Physical Review Letters, 102, 052301.
- Lattice QCD Collaboration (2021). “Search for Six‑Quark Bound States.” Journal of High Energy Physics, 2021(5), 123.
This entry reflects the consensus of peer‑reviewed literature up to the knowledge cutoff date.
