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The Many Ways to Search for Dark Matter
They sought it with thimbles, they sought it with care,
They pursued it with forks and hope,
They threatened its life with a railway-share,
They charmed it with smiles and soap.
Dark matter has long been a popular subject choice for a public talk on particle physics or astronomy. Not only is it genuinely one of the biggest mysteries in modern science, but it is also a great story. The astronomical evidence that the majority of the galaxy is made from some unknown invisible substance is overwhelming. The theory that this missing matter consists of a new type of particle, only interacting weakly with ordinary matter, is the frontrunner explanation. It falls to particle physicists to test this hypothesis by searching for dark matter particles—a challenge which we accept with relish. Searching for new particles is our favourite sport, and this one is particularly big game. Unlike the short-lived particles discovered in recent decades, which decay in a tiny fraction of a second, a dark matter particle must be stable. These things have been hanging around the universe, pulling galaxies into shape, since shortly after the Big Bang. A verified detection would be a really big prestigious discovery for the team that did it.
The classic way to search for new particles is with an accelerator such as the Large Hadron Collider. Take two high-energy proton beams and steer them together so they wham into one another, briefly reproducing the conditions of a fraction of a second after the Big Bang. Then use a cathedral-sized detector to search for the signature of something new in all the particle debris flying out from the interaction point. This was how the Higgs Boson was discovered in 2012. Searching for a dark matter particle is a little different as it is highly unlikely to interact with your detector. Instead you infer its existence from the absence of other stuff. You add up the momenta of every particle you can see and look for a dark-matter-shaped hole in the data. The teams at CERN have scrutinised their data sets for every possible flavour of dark matter. So far without a positive result.
Yet the LHC is not actually searching for galactic dark matter, they are aiming to make their own. While it could, in principle, show that a particle with all the characteristics of an ideal dark matter candidate exists in nature, it would take a more passive experiment to show that it accounts for the missing matter in the galaxy. A direct dark matter search is basically just a sensitive particle detector in a deep underground laboratory. If you still see something there, that’s your dark matter signal—assuming you can rule out anything else. Even deep underground, traces of uranium in the rock walls will produce gamma rays and neutrons. Dark matter searches have developed sophisticated detectors to distinguish the different types of particles and thus veto the radioactive background.
Back when I worked on the CRESST dark matter search as a PhD student the cryogenic detectors we used were a thing of beauty. Crystal of sapphire cooled to a cryogenic temperatures. A particle hit would deposit enough heat to raise the temperature by a few millionths of a degree, measured by a superconducting thermometer. The current leading experiments use a different technology, looking for flashes of light produced when particles interact in large tanks of liquid xenon.
In addition to particle colliders and direct detectors, there is a third way—indirect detection experiments scan the cosmos for a tell-tale sign that ‘dark matter was here’. Dark matter is weakly interacting, not only with ordinary matter, but also with itself. A lonely dark particle, created shortly after the Big Bang, may have been circling the galaxy for billions of years without meeting another. Yet two dark matter particles will occasionally collide, and when they do, they annihilate with a similar bang to the collision of two protons at the LHC, sending gamma rays and exotic particles out into space.
Indirect searches include space projects like the Fermi telescope, which saw an excess of gamma rays coming from the Milky Way, and the PAMELA satellite, which saw an excess of positrons. Some said this was a sign of dark matter. The trouble is that we know plenty of other things producing gamma rays and positrons in galaxy, and there are probably quite a few we don’t know. We just don’t understand the background well enough to say for sure if this is a signal.
A more convincing indirect signal could come not from space, but from the centre of the Earth. While billions of dark particles zip through the Earth every day without hitting a single atom, a few will bounce off a silicon or iron nucleus deep in the core of our planet. Having lost momentum in this encounter, they may then lack the speed to escape the Earth, and end up stuck orbiting around the centre of our planets. Over billions of years, a large number could end up inside the Earth, losing more energy through occasional interactions with atoms, until they fall to the centre, where eventually two of them meet and annihilate.
We would never see any gamma rays produced by such a dark annihilation at the centre of the Earth, but there is another possibility. It could produce a pair of neutrinos—those elusive light particles, weakly interacting enough to zip straight through the Earth, but detectable with a big enough neutrino telescope.
The IceCube project at the South Pole Station aim to do just this using strings of light detectors, lowered into holes in the ice, to visualise the tiny flashes of light produced when a high energy neutrino hits the ice sheet. If enough were seen to be coming from the centre of the Earth, this would be a pretty convincing sign of dark matter. The chain of events leading to this sight may seem complex, but no one has thought of another process which could produce such a signal.
There are many ways to search for dark matter. One thing they have in common is that none of them have seen a convincing signal. Aside from a few false positives, the quest to detect dark matter has just given us a decades-long string of null results. Meanwhile the astronomical evidence has mounted up. The astronomers are more convinced than ever that it is out there, but exactly what it is remains a mystery for now.