So with the Higgs particle sighted and the gongs distributed, physics seems finally ready to move on. Unless the Higgs had remained elusive, or had turned out to have much more mass than theories predicted, it was always going to be the end of a story: the final piece of a puzzle assembled over the past several decades. But now the hope is that the Large Hadron Collider, and several other big machines and experiments worldwide, will be able to open a new book, containing physics that we don’t yet understand at all. And the first chapter seems likely to be all about dark matter.
Depending on how you look at it, this is one of the most exciting or the most frightening problems facing physicists today. We have “known” about dark matter for around 80 years, and yet we still don’t have a clue what it is. And this is a pretty big problem, because there seems to be more than five times as much dark matter as there is ordinary matter in the universe.
It’s necessary to invoke dark matter to explain why rotating galaxies don’t fly apart: there’s not enough visible matter to hold them together by gravity, and so some additional, unseen mass appears to be essential to fulfil that role. But it must be deeply strange stuff—since it apparently doesn’t emit or absorb light or any other electromagnetic radiation (whence “dark”), it can’t be composed of any of the fundamental subatomic particles known so far. There are several other astronomical observations that support the existence of dark matter, but so far theories about what it might consist of are pretty much ad hoc guesses.
Take the current favourite: particles called WIMPs, which stands for weakly interacting massive particles. Pull that technical moniker apart and you’re left with little more than a tautology, a bland restatement of the fact that we know dark matter must have mass but can’t interact (or barely) in any other way with light or regular matter.
It’s that “barely” on which hopes are pinned for detecting the stuff. Perhaps, just once in a blue moon, a WIMP careening through space does bump into common-or-garden atoms, and so discloses clues about its identity. The idea here is that, as well as gravity, WIMPs might also respond to another of the four fundamental forces of nature, called the weak nuclear force—the most exotic and hardest to explain of the forces. An atom knocked by a WIMP should emit light, which could be detected by sensitive cameras. To hope to see such a rare event in an experiment on earth, it’s necessary to exclude all other kinds of colliding cosmic particles, such as cosmic rays, which is why detectors hoping to spot a WIMP are typically housed deep underground.
One such, called LUX, sits at the foot of a 1500m mineshaft in the Black Hills of South Dakota, and has just announced the results of its first three months of WIMP-hunting. LUX stands for Large Underground Xenon experiment, because it seeks WIMP collisions within a cylinder filled with liquid xenon, and it is the most sensitive of the dark-matter detectors currently operating.
The result? Nothing. Not a single glimmer of a dark-matter atom-crash. But this tells us something worth knowing, which is that previous claims by other experiments, such as the Cryogenic Dark Matter Search in a Minnesota mine, to have seen potential dark-matter events probably now have to be rejected. What’s more, every time a dark-matter experiment fails to see anything, we discover more about where not to look: the possibilities are narrowed.
The LUX results are the highest-profile in a flurry of recent reports from dark-matter experiments. An experiment called DAMIC has just described early test runs at the underground SNOLAB laboratory in a mine near Sudbury in Canada, which hosts a variety of detectors for exotic particles, although the full experiment won’t be operating until next year. And a detector called the Alpha Magnetic Spectrometer (AMS) carried on board the International Space Station can spot the antimatter particles called positrons that should be produced when two WIMPs collide and annihilate. In April AMS reported a mysterious signal that might have—possibly, just about—been “consistent” (as they say) with positrons from dark-matter annihilation, but could also have more mundane explanations. LUX now makes the latter interpretation by far the most likely, although an international group of researchers has just clarified the constraints the AMS data place on what dark-matter can and can’t be like.
What now? LUX has plenty of searching still to do over the next two years. It’s even possible that dark-matter particles might be produced in the high-energy collisions of the LHC. But it is also possible that we’ve been barking up the wrong tree after all—for example, that what we think is dark matter is in fact a symptom of some other, unguessed physical principle. We’re still literally groping around in the dark.