New candidate particle for dark matter is one we've already found

Dark matter is believed to outnumber regular matter by a ratio of five-to-one, but so far it’s never been directly detected. Now, nuclear physicists at the University of York have proposed a new candidate particle that might make up the strange stuff – and we’ve already found it.

Almost a century of observations and calculations tell us that there’s far more mass in the universe than just the stuff we can see. This unknown mass seems to interact with regular matter only through gravity, and doesn’t emit, absorb or reflect any light – hence the dark matter moniker.

Exactly what dark matter is made of has remained a mystery for decades, but physicists have proposed many candidate particles. Suggestions include axions, dark photons, weakly-interacting massive particles (WIMPs), superheavy gravitinos, “Macro” particles with the mass of a dwarf planet, and scalar particles older than the Big Bang.

But the thing is, all of these particles are hypothetical and despite extensive experiments, none of them have ever been confirmed to exist. And that’s what makes the new hypothesis so intriguing – this newly-proposed candidate has already been detected.

The particle is technically referred to as d*(2380), or the d-star hexaquark, and it was discovered in experiments in 2014. Normally, protons and neutrons are made up of sets of three quarks (fundamental particles), but this new particle contains sets of six quarks.

That makes the d-star hexaquark a boson, and these clump together in unusual ways under certain conditions. When chilled to almost absolute zero, for instance, they form a Bose-Einstein condensate (BEC), a state of matter where particles begin to act like one big “superatom.”

And this, according to the new hypothesis, could be an explanation for dark matter. The York physicists hypothesize that soon after the Big Bang, conditions would have been just right for d-star hexaquarks to gather as Bose-Einstein condensates, in large-enough amounts to create the effects that dark matter is known for.

“Our first calculations indicate that condensates of d-stars are a feasible new candidate for dark matter,” says Daniel Watts, co-author of the study. “This new result is particularly exciting since it doesn’t require any concepts that are new to physics.”

Of course, at this stage the idea is still hypothetical as well, and further work is needed. The researchers plan to test their theory in the lab, and begin searching the skies for the kinds of signals that may indicate d-star BECs are floating around out there.

“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,” says Mikhail Bashkanov, co-author of the study. “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 research was published in the Journal of Physics G Letters.

Source: University of York