There’s far more to the Universe than meets the eye. We now know that the vast majority of its mass is in the form of some exotic, invisible material called dark matter that pervades space, binding galaxies together by exerting a gravitational force on the matter that we can see. Take this ‘cosmic glue’ away and the stars in galaxies like our Milky Way would fly apart like sparks from a Catherine wheel. So why, after several decades of searching with super-sensitive underground detectors, are we still waiting for the first definitive signal of dark matter?
The trouble with dark matter is that it’s immune to two of the other three forces of nature (the strong and electromagnetic forces), and only feels the final force (the weak force) very feebly, if at all (for a nice introduction to particles and forces, see the first few pages of this booklet). You are only able to read this blog post because the photons being emitted from your screen interact with electrons in your retinas via the electromagnetic force, transferring some energy as they do so. Dark matter, on the other hand, is extremely reluctant to interact with anything, to the point where it happily streams through the Earth, barely noticing our presence at all. It’s almost completely indifferent to us, which is kind of annoying, come to think of it, for people like me who have spent years, and in some cases whole careers, trying to pin down the elusive stuff. On the other hand, it’s one of the biggest unsolved mysteries in science, so whoever catches the first glimpse is odds-on for a Nobel prize.
Just as the eye detects the energy transferred in photon interactions, so we design our experiments to measure the energy transferred in dark matter interactions. The problem is, if you fire up your detector in an ordinary lab you’ll see plenty of interaction ‘events’, but they will almost all be caused by cosmic ray muons: charged particles generated around 15 km above the Earth’s surface where the solar wind impacts the upper atmosphere.
This is where dark matter’s stand-offishness comes into its own. Remember how it hardly notices that the Earth is there? That means that the amount of dark matter streaming through a given volume (say, a 1 m3 particle detector, for example) is pretty much the same underground as it is on the surface of the Earth, on the Moon, or in interstellar space. Unlike dark matter, cosmic ray muons do notice the presence of the Earth, and are absorbed by the layers of rock, dinosaur bones, and fragments of Roman pottery that make up the Earth’s crust (geology isn’t my strong suit). So, go deep enough underground and you’re no longer swamped by these annoying ‘background’ events from cosmic ray muons, yet there should still be dark matter around for you to detect.
For this reason, all of the world’s dark matter search experiments are housed in deep underground laboratories, from a disused gold mine in South Dakota to a road tunnel beneath the Apennine Mountains, to a nuclear waste storage facility in New Mexico. We have one in the UK too. It’s called the Boulby Underground Science Facility, and it’s part of a working mine just up the coast from Whitby in North East England.
Today, Boulby has a diverse science program ranging from astrobiology to Mars rovers, but it cut its teeth back in the early noughties as a world-class dark matter search facility, playing host to the ZEPLIN and DRIFT experiments. Whilst ZEPLIN scientists pioneered the two-phase xenon technology that is currently (as of October 2014) the world’s most sensitive method of searching for dark matter, the DRIFT collaboration took a different approach. Rather than trying to measure just the energy deposited in dark matter interactions, DRIFT seeks to also measure the trajectory of the incoming dark matter.
The theory goes that the Galaxy’s ‘disc’ of visible matter (including us) is rotating through a stationary, roughly spherical ‘halo‘ of dark matter surrounding and pervading the Galaxy. So, as we travel through this halo, we should expect to see an excess of events coming from a patch of the sky corresponding to the solar system’s direction of travel, which happens to be in the constellation Cygnus.
Unfortunately, contrary to sensational claims that you may have seen in the news, there is no consensus in the community that any detector has yet caught a glimpse of dark matter. There have been hints, but these invariably end up being excluded by more sensitive experiments, or explained by sources of background events. Not the cosmic ray muons we met earlier, but more mundane terrestrial processes like the decay of trace radioactive impurities in the detector. For this reason, a huge amount of effort goes into screening out potentially radioactive materials when building a dark matter detector. In the absence of a positive detection, the best we have been able to do so far is set limits on the willingness of dark matter to interact with ordinary matter. The frustrating thing is, it must be out there! It just seems that Nature has chosen to make it almost completely indifferent to our presence, other than through the force of gravity.
As winter draws in, you may find yourself looking up on a crisp, cloudless night – especially with bonfire night just around the corner. If you’re in a dark spot and the fireworks haven’t started yet, you may be able to make out the faint streak of the Milky Way across the night sky. If you do, spare a thought for the dark matter that’s up there, and down here, quietly holding it all together.
About me: Stephen Sadler
I’m a Post-Doc at the University of Sheffield. Underground I spend my time working on a directional dark matter search experiment called DRIFT (not the memetic kind!). On the surface I’m keen on mountain biking, gaming and brewing (occasionally drinkable) wine. Find me on Twitter: @paddlesacks
Featured image: Credit goes to IgnitePyro for the stunning Catherine Wheel shot