So what exactly is a WIMP? In the current physics models, these particles interact through gravity but have little response—or none at all—to the other forces. There are a few dark-matter candidates besides WIMPs, but all of them have to be similarly slow-moving to produce the kind of clumpy distribution that astronomers infer from dark matter’s gravitational pull. Because these particles rarely interact with ordinary matter, they would have to be electrically neutral and hence invisible to light, X-rays, or any other form of electromagnetic radiation. Theorists suggest that WIMPs are anywhere from 40 to 1,000 times as massive as protons. Clouds of these particles seem to embrace galaxy clusters in a large sphere, and they seemingly move unimpeded through the universe. Billions may be passing through your body every instant, but since they seldom hit anything and have no charge, they would provide no evidence of either their arrival or their departure. They are called “dark” for a reason.
What makes WIMPs the most attractive of the dark-matter candidates is that their existence would elegantly resolve not only the missing mass problem but also a shortcoming in the standard model of particle physics. That model describes all the particles and forces in the universe but does not adequately explain the theory of mass. To fix this, theorists have introduced the concept of supersymmetry, in which each matter particle, known as a fermion, and each force particle, known as a boson, have large-mass counterparts, called superpartners.
Click image to enlarge
Collisions between dark-matter particles
known as WIMPs, and ordinary atoms
should be detectable because only WIMPs
and neutrons can pierce an atom's cloud
of electrons with enough momentum to make
nucleus vibrate, creating a tiny amount
of heat. A neutron will rebound hitting
multiple atoms, while a WIMP
will interact with just one atom
producing a telltale signal.
Image courtesy of Michael Attisha
In this framework the lightest neutral superpartner particle—with a mass at least 50 times that of a proton—is the neutralino. Particle physicists predict that the number of neutralinos left over from the Big Bang roughly matches the expected amount of dark matter, making the neutralino the prime WIMP candidate. “In the 1980s you show that dark matter is really there,” Turner says. “Then you describe the particles that could explain the discrepancies in the standard model, and then you find that those particles have the right properties to be dark matter. That can’t be a coincidence.”
There are now about 10 teams of scientists huddled in underground labs around the world trying to catch a WIMP. One of them, at the University of Rome, even claimed success in 2000, sparking a brief outbreak of elation in the physics community—until no one else could confirm the result. All the experiments rely on the same principle: detecting WIMPs on the rare occasions when they collide with an atomic nucleus. The hunt for the WIMP is a global competition, and competition being what it is, some other physicists think they have come up with a better WIMP trap. Most promising are detectors that use liquid noble gases—unreactive gases like xenon and argon—that will emit a flash of light when a WIMP strikes.
Richard Gaitskell of Brown University, a former CDMS team member who currently participates in XENON, a liquid gas experiment in an underground laboratory in Italy, says that inert gases have a couple of advantages over CDMS. Since the experiment doesn’t rely on detecting vibrations, the XENON team doesn’t need to worry about quieting the target atoms by cooling them to near-absolute zero. Xenon, a gas at room temperature, turns to liquid at a relatively balmy –148°F. There is no need to go lower. Also, as a single nearly spherical liquid target, the xenon system has a much smaller surface-to-volume ratio than multiple germanium detectors and therefore is less susceptible to noise. Finally, scaling up the size of the detector, and thereby enhancing the probability of success, is much easier with a liquid. “We’re capable of not only equaling but also surpassing CDMS,” says Gaitskell. “You need multiple technologies so you can check each other’s results.”
The CDMS researchers are conceding nothing. “Right now we are better than anyone else,” says Sadoulet, although he acknowledges, “maybe not for long. At the moment, the noble liquids are breathing down our necks.” And Soudan will not be the last word in the CDMS team’s dark-matter search. Sadoulet and company are already hatching plans to build larger detectors and take them even deeper—6,800 feet down in the Sudbury Neutrino Observatory in Ontario, Canada.
“Every time we have a new stage, we’re hoping to find WIMPs, but it’s really only in the last year that we have really enhanced our chances,” Sadoulet says. If CDMS finds dark-matter particles, “there will be a rush to increase the mass of detectors and cross-check with other technologies to make sure they are WIMPs.”
Even with just a few detection events—“maybe five,” Sadoulet suggests—CDMS should be able to make a lower limit estimate of WIMP mass and get an idea of the interaction rate, giving us a first glimpse at the properties of the cosmic dark stuff. Above all, a detection would prove that physicists’ dark-matter theories are on track. Sadoulet sees good odds that it will happen within the next couple of years.
Then a new hunt will begin. By building larger detectors to get more hits, researchers will be better able to characterize WIMPs. They will also want to know how dark-matter particles move through our planet and our galaxy. “You’ll want to look at directionality, see where the WIMPs are coming from,” Sadoulet says.
The practical value of finding dark matter would probably be minimal, he admits. The philosophical implications, on the other hand, would be mind-boggling. “If you found one, you’d want to find more,” Sadoulet says. “I would like very much to know what makes up most of the universe.”
