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Tetraquarks

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Pentaquarks

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Hexaquarks

Progress in searches for exotic forms of matter

The matter in the visible universe is comprised mainly of protons and neutrons, bound systems each containing 3 quarks. For almost half a century the only bound system containing 6-quarks established in nature was the deuteron, a molecule composed of a proton and a neutron with aligned spins. Recent work led at Edinburgh established a new system of 6-quarks, the d* resonance, which has an unusually long lifetime.

In the case of the deuteron the particle is only stable (bound) if the spins of constituents are aligned. However one can also form a “demon deuteron” – a proton-neutron system with the spins anti-aligned which for the case of the deuteron is almost stable (unbound by 66 keV only). If the “demon deuteron” would be bound, only neutron stars and black holes would exist in our universe.

For the new d* it is of fundamental importance to search for its “demon state” with mirrored-spin. This tells us new information about the nature of multiquark states. There should be 7 “demon d*” particles with degenerate mass and with electric charges ranges from -2 to +4. The demon d* with the highest charge would consist of six u-quarks – the maximum number of quarks which can be bound according to our current understanding of the theory of the strong interaction.

In Phys.Lett. B762 (2016) 455-461 we report about a search for the “demon d*” and set stringent upper limits on its existence. We conclude that the demon d* is unbound, providing important new constraints on the poorly understood physics of multiquark states.

This work has been supported by STFC (ST/L00478X/1)