The D-Star Hexaquark

Moving on from medical applications, we are now going to explore an exciting new discovery in particle physics. The d-star hexaquark, described by Dr Mikhail Bashkanov in the video above, is a new form of matter composed of six light quarks – the same quarks that usually combine in trios to make up protons and neutrons.

D-Star Hexaquark Discovery

An experiment looking for the particle was led by the University of York using the Crystal Ball detector at the Mainz Microtron (known as MAMI). MAMI is a particle accelerator located at the Johannes Gutenberg University in Mainz, Germany. Like many other particle accelerators, MAMI is used by physicists from around the world to investigate nuclear and particle physics.

The experiment searched for the d-star hexaquarks by focusing an intense gamma ray beam on a liquid deuterium target. You may remember from week 1 that deuterium is an isotope of hydrogen with one proton and one neutron in the nucleus (called a deuteron). Deuterium is found naturally in seawater, where it constitutes approximately 0.0153% of hydrogen, or can be concentrated to form so-called heavy water.

Formation of D-Star Hexaquarks

Hitting a deuteron with a photon causes the deuterium nucleus to split into a proton and a neutron in a process called photodisintegration. By using sufficiently high energy gamma ray photons, the quark substructure of the deuterons can be investigated. By measuring the final state of the protons and neutrons from the photodisintegration of the deuterium nuclei, it is possible to determine that d* hexaquarks were formed during this process. This is shown in the diagram below:

The existence of the d-star hexaquark was inferred by observing the degree of spin polarisation of the outgoing proton and neutron. For the first time, both were almost completely polarised at photon energies corresponding to the mass of the d-star hexaquark. This cannot be explained by current conventional theories. The results provide important new information for the emerging field of multiquark states and potentially for astrophysics, where the d-star hexaquark may play an important role in neutron stars and even provide a new candidate for the mysterious dark matter. We will look at these possible roles for the d-star hexaquark in the following articles and videos.

You can read more about the experiment in Signatures of the d*(2380) Hexaquark in d (γ,p→n), published in the APS Physics, Physical Review Letters (2020).