Internal structure of pentaquarks and exotic hadrons

Partly settled. Some of it is understood, some is still argued about.

The quark model allowed for exotic combinations beyond the usual three-quark baryons and quark-antiquark mesons, but no five-quark 'pentaquark' state was convincingly observed until the LHCb collaboration reported strong evidence (9-sigma and 12-sigma significances) in July 2015, later refined into three separate states (Pc(4312), Pc(4440), Pc(4457)) with a larger 2019 dataset. Existence is now well established and widely accepted, but whether these are compact five-quark bound states or loosely bound meson-baryon 'molecules' — and what other quark combinations can form similarly — remains an open theoretical and experimental question as of the mid-2020s.

Physicists spent fifty years looking for particles made of five quarks stuck together, found them, and immediately discovered they don't yet know what 'stuck together' really means for these strange new objects.

Explanations that circulated

• That the internal structure question is fully resolved — while existence is confirmed (2015, refined 2019), whether these states are compact diquark-triquark objects or loosely bound hadronic molecules remains actively debated

References

• Observation of J/psi p Resonances Consistent with Pentaquark States in Lambda_b0 -> J/psi K- p Decays, LHCb Collaboration, Physical Review Letters, 2015
• LHCb bags another pentaquark, Physics World staff, Physics World, 2019