The concept of dark matter was first introduced in the 1970s by Vera Rubin, Kent Ford, and Ken Freeman and it is now key to many of our theories. It's thought dark matter accounts for around 85 percent of the matter in the universe. Despite the many decades of work exploring it, however, scientists are yet to find exactly what dark matter is.
We know that it interacts with gravity (because of its gravitational effect on the things we can measure) but not with light (because we can't see it directly), and strong evidence suggests that it is actually a substance, rather than an issue with our laws of physics. Speculation continues on what particles make up dark matter, and researchers at the University of York have just suggested a new one, a hexaquark.
A hexaquark is a particle made of six quarks – one of the fundamental particles in physics, and the constituent bits of neutron and protons. Each of them, in turn, is made of three quarks each (two up and one down quark for the proton, and two down and one up for the neutron). This particular hexaquark, first detected in 2014, is called d*(2380) or d-star for short, and it decays in just a fraction of a second.
In a new paper published in the Journal of Physics G: Nuclear and Particle Physics, the researchers at York discuss some theoretical properties of d-star, which makes it an attractive candidate for what formed dark matter. D-star hexaquarks are bosons. This property allows them to assemble in a Bose-Einstein Condensate, the so-called fifth state of matter. In this state, quantum phenomena become apparent macroscopically – large enough to be visible by the naked eye. The researchers think Bose-Einstein condensates formed from the d-star hexaquark could have formed in the early universe on a large enough scale to make it a contender for dark matter.
“The origin of dark matter in the universe is one of the biggest questions in science and one that, until now, has drawn a blank,” co-author Professor Daniel Watts, said in a statement. “Our first calculations indicate that condensates of d-stars are a feasible new candidate for dark matter. This new result is particularly exciting since it doesn’t require any concepts that are new to physics.”
The researchers believe that the early universe could have had the right conditions to form enough d-star hexaquarks to be a valid dark matter candidate. However, this field has had plenty of candidates that were thought promising and since been discarded.
“The next step to establish this new dark matter candidate will be to obtain a better understanding of how the d-stars interact – when do they attract and when do they repel each other,” co-author Dr Mikhail Bashkanov added. “We are leading new measurements to create d-stars inside an atomic nucleus and see if their properties are different to when they are in free space.”
The team will have to establish a lot more properties of the hexaquark to make a truly compelling case and it will require testable predictions. In their paper, they mention a few that could put limits on the existence of a d-star condensate.