Nature News | Adam Mann | 2011 Mar 23
The race to detect dark matter has yielded mostly confusion. But the larger, more sensitive detectors being built could change that picture soon.
For a substance that is utterly invisible, dark matter does a remarkably good job of making its presence felt. Astronomers have been compiling evidence for it since the 1930s, tracing how it shapes galaxies, galaxy clusters and even bigger cosmic structures by the inexorable force of its gravity. Although its real nature is unknown, dark matter seems to outweigh the ordinary matter visible in stars and galaxies by roughly 5.5 to 1.
Down here on Earth, however, physicists struggling to answer the 'what is it?' question often feel like they're chasing a ghost. Certainly, their detectors have been giving them a lot of strange and contradictory results. Two experiments are independently seeing what seems to be a flux of dark matter streaming through their apparatus. Another detector may have seen a handful of dark-matter particles last year — although the experimenters dismiss them as background noise. And yet another experiment has found no evidence for dark matter at all.
Fortunately, this confusion is likely to be temporary. Dark-matter detectors are roughly 1,000 times more sensitive to ultra-rare events than they were 20 years ago, and that should increase by another factor of 100 over the next decade, as physicists build bigger detectors and become more skilled at suppressing the background noise than can be confused with genuine signals (See 'Dark-matter detectors'). "It would not be surprising if a year from now someone stood up and said we have done it, we've detected dark matter," says Sean Carroll, a theoretical physicist at the California Institute of Technology in Pasadena. Other physicists give a more cautious estimate of five to ten years. Nonetheless, there is a palpable sense that the field is on the verge of something big.
Most of the attempts to detect dark matter directly have started from the assumption that the stuff is a haze of weakly interacting, massive particles (WIMPs) left over from the Big Bang. The 'massive' part would explain the gravity. And the 'weakly interacting' part would explain the invisibility: the WIMPs would flow through stars, planets and people in untold numbers, almost never hitting anything.
That assumption dictates the basic detection strategy: bring together a large target mass of material; put it deep underground to shield it from cosmic rays and other radiation that could produce misleading signals; then measure the recoil energy when a dark-matter particle finally hits an ordinary nucleus. The larger the mass of material, the more likely it is that a dark-matter particle will hit something.
Beyond those basics, setting up such an experiment requires a certain amount of guesswork. To have a significant recoil effect, for example, researchers need a target nucleus of roughly the same mass as the dark-matter particle they are seeking. It's like watching for an invisible pool ball, says Jonathan Feng, a particle physicist at the University of California, Irvine. If the target nucleus is the equivalent of a bowling ball, the impact will barely move it. If, on the other hand, the target is the equivalent of a ping-pong ball, it will hardly be capable of deflecting the dark-matter particle, and so again there will be little energy transferred. What you want is another pool ball, Feng says.
- Supersymmetrical WIMPs …
- Pure and simple …
- Total annihilation …
