Two recent experiments limit physicists’ favorite candidate for the elusive and invisible matter lurking in the universe.
It’s been a bad couple of weeks for wimps.
Weakly interacting massive particles (WIMPs) are the top candidates for dark matter, the invisible stuff that makes up about 84% of the universe’s matter. But two recent experiments designed to sniff out the elusive particles have come up empty-handed, calling previously promising results into doubt.
WIMP-y Hints of Dark Matter
By definition, dark matter doesn’t interact with light, so not only is it dark, it’s transparent too. And while we know it interacts with gravity, that interaction leaves only indirect evidence of its existence, such as its effect on galaxy rotation.
But WIMP theory says dark matter particles should also interact via the weak force, a fundamental force that governs nature on a subatomic level. So a WIMP particle will very rarely smash into a heavy nucleus, leaving a detectable signal. The chance for a direct hit is very, very low, but underground detectors stacked full of cold silicon, germanium, or xenon have the best odds for success.
Several detectors have claimed to see hints of WIMP interactions (but not actual detections, which would merit a flight to Stockholm). Some of these, such as the CDMS-II, CRESST, and CoGeNT detectors, have seen a possible signal from a WIMP particle with a mass near 10 GeV — a signal important enough to announce, but not statistically significant enough to be named a real detection. Another detector, DAMA/LIBRA, has seen a much stronger signal, but not everybody is convinced it comes from WIMPs.
But not every experiment has seen positive signs of dark matter. The XENON-100 experiment had already cast some doubts on previous results because it didn’t see any WIMP interactions. But this result wasn’t anything physicists couldn’t theoretically massage away. For example, some suggested that WIMPs might interact differently with the silicon nuclei of CDMS-II’s detector than with the xenon in XENON-100.
A Null Result
Now a new experiment called LUX (short for Large Underground Xenon) may have barred that way out. Richard Gaitskell (Brown University) and Dan McKinsey (Yale University) announced today the results from LUX’s first 85 days of operation. The new detector is 20 times more sensitive than XENON-100. If, say, the CDMS-II detections were real, the researchers say LUX’s six-foot-tall titanium tank of liquid xenon ought to have interacted with some 1,550 WIMPs.
Instead, it saw zero.
“It does not appear possible to reconcile [CDMS-II and LUX],” Gaitskell says. Claiming that WIMPs interact differently with some elements than others won’t work this time. “LUX’s results are in direct conflict with DAMA, CoGeNT, and CRESST.”
(The graph at right says it all — the take-away is that the blue line, which marks the upper limit set by the LUX experiment, lies below all the detections seen in previous experiments, marked by colorful blobs.)
Dan Hooper, a dark matter expert at Fermi National Accelerator Laboratory who was not involved in the LUX study, agrees: "It would be fair to say that dark matter interpretations of the CoGeNT and CDMS data seem pretty unlikely to me in light of the new results from LUX."
"I would be lying if I said I was not more than a little disappointed," he adds.
If LUX’s results rule out all previous detections, then what were the other detectors seeing? For the barely there results reported by CDMS-II, CoGeNT, and CRESST, Gaitskell says it’s difficult to trust data at the margins. “I think unfortunately what we’ve seen is a series of partial results and experiments where they’re reporting right at the limit of their capabilities.”
But DAMA’s results are by no means marginal — on the contrary, they’re almost impossible to explain as a statistical fluke. “There is no doubt that DAMA is measuring something,” Gaitskell explains. But the equipment isn’t precise enough to point to a cause. “The challenge there is to determine whether it is of an astrophysical nature, or whether it comes from some more mundane, anthropological, or just earthbound explanation.”
Though LUX appears to contradict the hints of direct detection seen so far, Gaitskell cautions, “one should never say never. Theorists are very inventive.”
Even if all previous WIMP hints are wrong, there are still many more models to test, up to 1,000 times below the interactions LUX is sensitive to. “We will be searching for WIMPs for a while longer,” Gaitskell says, undeterred. “They remain the favored quarry.”
Missing Gamma Rays
Direct detections of a slippery particle that avoids ordinary matter like the plague is difficult at best. So some scientists prefer a more indirect approach. The standard WIMP model says the particles are their own anti-particles, so if WIMPs meet their siblings in deep space, they should unleash a flurry of gamma rays at an energy corresponding to the mass of the individual particles.
NASA’s Fermi Gamma-Ray Space Telescope has been scouring the sky to look for this annihilation signature, focusing especially on dwarf galaxies near the Milky Way. Astronomers think these galaxies have much more dark matter than ordinary matter, which should make them decently bright gamma-ray sources. But the Fermi-LAT collaboration recently submitted a study to the journal Physical Review D that reports no significant gamma-ray emission from 25 nearby dwarfs.
Unlike LUX, the Fermi observations don’t contradict the hints seen from CDMS-II and other experiments, at least for now. That’s because it’s not clear how the probability of WIMP particles smashing into each other relates to the probability of WIMP particles smashing into ordinary matter. But that will change as Fermi continues to collect gamma-ray observations — if dark matter really is made of WIMPs, the collaboration expects to see a signal soon.