Why Are Scientists Going Underground To Find Dark Matter?

There are a few reasons why putting your dark matter detector deep beneath the Earth is good. Some have to do with the nature of dark matter. The belief that it is out there comes from the fact that galaxies move like they have a lot more mass than they appear to, so the dark matter would interact gravitationally but not with light. If dark matter is indeed made of particles, said particles could then occasionally hit an atom – and if this atom was inside a detector, it would cause a flash of light.

“It really is not going to be any of the particles we already know about. It’d be particles that behave similarly to neutrinos, which we know exist. Billions are passing through us all the time now, but it has to weigh more. They don’t weigh enough. So there are a range of particles of different masses from very light to very heavy.” Professor Sean Paling, Director of Boulby Underground Laboratory, told IFLScience.

But dark matter particles and neutrinos are not the only particles that are freely moving about: The surface of our planet is constantly bombarded by cosmic rays. Some originate from the Sun, others from energetic cosmic events such as supernovae or the neighborhood of black holes. As they hit the atmosphere, they cause a rain of secondary particles that can also enter detectors and hit atoms, creating flashes of light.

For this reason, those detectors need to be shielded, and the easiest way to do it is to place many hundreds of meters of rock between them and the surface of the planet. Boulby Underground Lab is run by the Science and Technology Facilities Council. It is located 1.1 kilometers (0.68 miles) below ground in Boulby Mine, a working polyhalite and salt mine – at that depth, the background from cosmic rays is one million times smaller.

“If you come down here, where we are now, we’ve got a kilometer, 1.1 kilometers above our head of rock that shields us from those cosmic rays,” Professor Paling continued. “All the theories suggest that these dark matter particles should be able to pass through [the rock]. So the dark matter particles should be here, but the cosmic rays should stay up there. So it’s a quiet environment to run these very sensitive particle detectors.”

Boulby is certainly not alone in using geology as protection against cosmic rays. A dark matter detector called XENON is located in the Italian Gran Sasso National Laboratory, deep beneath the tallest mountain in the Apennines. There is also the dark matter detector LZ experiment located in a former South Dakota mine. Similar in approach, but different in target, is the Super-Kamiokande neutrino detector found under Mount Ikeno in Japan.

Those detectors have a connection with Boulby beyond their buried locations. Beyond cosmic rays, there is another source of possible confusion: Many materials, from some rocks to bananas (and even humans), are naturally radioactive. 

“Radio purity is a big problem for scientists because they can’t get their dark matter detectors with exactly no background from the detector itself,” Beth Green, Laboratory Technician Apprentice at Boulby Underground Laboratory, told IFLScience.

Boulby helps establish the radioactivity level for the material that builds up those detectors. They have several experiments to do so, and their results shape particle detectors and their science across the world.

Boulby hopes to be the home of the next international dark matter detector, but the facility does more than search for dark matter. It is involved in nuclear safety, as well as being a place to test innovative forms of energy storage and prepare both tech and people for going to other worlds.

Learn more about this fascinating lab and the search for dark matter in our mini-documentary here or above.