Dark matter is notoriously elusive. For decades we have searched for its existence, and while we can infer its presence in the universe from its effect at a galactic scale, finding direct evidence for it has been a bit more difficult. But what if it has been hiding under our feet this whole time?
That’s what a team of researchers are proposing in a paper available on the pre-print server arXiv. They say that it might be possible to see the effects of theorised dark matter particles called weakly interacting massive particles (WIMPs) on ancient rocks right here on Earth. And they’ve got an idea to prove it.
Despite being suspected of making up 85 per cent of matter in the universe, dark matter’s lack of interaction with regular matter has made finding direct evidence for its existence very difficult. Most of our searches, such as at the Gran Sasso Underground Laboratory in Italy, at SNOLAB in Canada, and numerous other labs around the world, have been unsuccessful so far.
Those detectors typically look for interactions over several years, but Baum and his colleagues suggest so-called paleo-detectors could record interactions over hundreds of millions of years. In certain minerals such as halite, found deep underground, a WIMP hitting a nucleus would cause it to recoil, producing a noticeable trace – or “damage track” – where the nucleus had been bumped out of position.
Such an effect would be tiny; the trace would be less than 1,000 nanometres in size, far too small for the human eye to see. But if you used an advanced technique such as helium ion microscopy, you could see the remnants of the event long after it had happened.
No one knows for sure yet though, because these techniques have only become possible in the last decade. A similar idea was proposed in the late 1990s, using the mineral mica to try and see traces of dark matter, but the technology simply wasn’t good enough then.
The other issue is background scattering. In June, the team released their first paper on this idea, but some scientists criticised the proposal because they said the researchers wouldn’t be able to tell if nuclei had been hit off course by a dark matter particle, or just some errant radiation.
In their latest paper, the team say the length of the damage track could solve this. WIMPs are expected to be relatively heavy compared to other particles. If the track is short, background sources could be at play. But if it’s long, the only thing that could explain that might be a weighty dark matter particle.
Within the next year, the researchers are hoping to perform a feasibility study to prove their technique works, showing that they can see traces not related to dark matter inside a material. Some time after that, if all goes well, they plan to actually try and find evidence for traces caused by WIMPs.