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The future for d* hexaquark research

Professor Dan Watts explains how the d* hexaquark might form into a Bose-Einstein condensate. Could it be a candidate for dark matter?
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The d* hexaquark provides a new way for the light quarks in the universe to combine in groups. So we’re familiar with the trios of quarks which make up the protons and neutrons, which condensed out of the early plasma of quarks and gluons in the early stages of the universe. These protons and neutrons, these trios, later formed into atomic nuclei and these now provide most of the mass of the visible universe. The protons or neutrons have an intrinsic quantum mechanical property associated with them, referred to as spin. Both these particles have a spin of one-half and belong to a class of particles referred to as fermions.
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So fermions cannot occupy a quantum state if it’s already filled, we say they obey the Pauli exclusion principle. That is why the nucleons, when they are bound in an atomic nucleus, stack up in the available quantized energy levels – they don’t all drop to the bottom. This is also analogous to how electrons, which are also Fermions, stack up in the quantized energy levels when bound in an atom. And this stacking of the electrons actually gives rise to the field of chemistry. So now, this hexaquark has spin three.
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So as well as being the highest spin for any particle in the ground state that we know of, it also has the important property that it’s an integral spin – so it’s a boson. So the hexaquark therefore gives a new way for light quarks to combine in a bosonic form, rather than a ferionic form. And when you have a collection of bosons, many hexaquarks, the behavior is different to the fermions. The bosons actually want to all get into the same state and have the same quantum numbers. When they’re in this state, they’re what is referred to as a Bose-Einstein condensate, and we know of Bose-Einstein condensates in many different systems, for example, helium-4 atoms.
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So establishing whether the d* hexaquarks can actually form these condensates is ongoing research. Early investigations indicate that if there are sufficient hexaquarks coming together, the resulting condensate – and the hexaquarks within the condensate – become stable and bound. The binding energy which was estimated is much higher than the electron-volt scale of the electrons in an atom, higher than the mega-electron-volt scale of the nucleons in a nucleus. It could be many thousands of MeV, or even at the tera-electron-volt scale. If you have such a condensate, for a particle or a photon of light to interact with it, it would need to possess sufficient energy to break a d* from the condensate and overcome this binding.
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An analogous situation arises when you consider why materials are transparent to visible light – for example, the light coming through a window or traveling through a diamond crystal. For the light to actually interact with the many, many electrons that it encounters as it passes through, it needs to have enough energy to boost that electron to a freely available quantum state. For transparent materials the energy required to do this cannot be achieved with visible light, so the photons travel straight through. So for the d* condensate, if the energy required is really on this extremely large tera-electron-volt scale, there’s actually very few particles roaming around the universe with sufficient energy to actually break that condensate.
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So the upshot is the condensates will be stable over the lifetime of the universe and provide an interesting new possibility for contributing to dark matter – speculative, but very interesting. We can’t create the condensates and the laboratory to test them, we can’t get enough d*s produced to make them, so astronomical observations are really an important way forward. Although they’re largely stable, we think there will be a much greater chance of breaking this hypothetical condensate dark matter if you’re near to an explosive event, an astrophysical event like a supernova, neutron star mergers, massive black holes, this kind of stuff. Around these objects, you have a much higher number of these high energy particles compared to what you get in typical space.
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So there’s the potential for them to be broken by these phenomena. So if the hexaquark is released from a condensate, it would then become unstable and we can look for the decay signatures from these released and decayed hexaquarks. From laboratory experiments we can work out the likely energy range of these decay products, and it’s very fortuitous and interesting that missions of gamma ray astronomy in space will be launched in the coming decade which are very sensitive to this energy region, and may potentially give us a chance to glimpse these hypothetical decays of hexaquark condensates.
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So we’re very interested to see these results, which will help us further the program – the exciting potential that we might see a glimpse of these hexaquark decays. So it’s going to be a very exciting coming decade.

Trying to identify candidates for dark matter is an ongoing and exciting field of astrophysics. There are still many possibilities, one of which is the d* hexaquark.

As Prof. Dan Watts discusses in the video above, in the conditions shortly after the Big Bang, many d* hexaquarks could have grouped together as the universe cooled and expanded to form the fifth state of matter – Bose-Einstein condensate. This would be stable over the lifetime of the universe and would behave in the way dark matter is currently observed to.

The next step in investigating this new dark matter candidate will be to obtain a better understanding of how the d* hexaquarks interact – when do they attract and when do they repel each other. University of York researchers are collaborating with scientists in Germany and the US to test their theory of dark matter and search for d* hexaquarks in the cosmos. They are leading on new measurements to create d* hexaquarks inside an atomic nucleus to see if their properties are different to when they are in free space, and are using data from telescopes such as the Fermi Gamma-ray Space Telescope to look for evidence of d* hexaquark Bose-Einstein condensates out in space.

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