Oh, dark matter, you saucy tease! Just when we think we’re zeroing in on finally figuring you out, you start flirting with some other experimental apparatus, just to keep everyone guessing.
That’s right, physicists are buzzing — again! — this week about new evidence for dark matter that seems to offer confirming evidence for a long-standing controversial claim that dark matter has not only been observed, but that it varies with the seasons.
These new results were presented by the University of Chicago’s Juan Collar, head of the CoGeNT experiment, at a meeting of the American Physical Society in Anaheim, California.
CoGeNT’s findings come just a few weeks after a different experiment, XENON100, released results that seemed to exclude the hints of dark matter that pop up from time to time, most notably that from Italy’s DAMA experiment, which detected tiny periodic fluctuations in the rate of events several years ago over a year-long time scale.
DAMA scientists think it’s dark matter. Other physicists aren’t so sure — although the signal is definitely there, it could be something else with a similar annual fluctuation — including Collar, which is why he set out to reproduce the DAMA experiment and prove them wrong.
The best laid plans, yadda, yadda, yadda…. On Monday, Collar (somewhat reluctantly, one assumes) admitted that the data from CoGeNT’s most recent run shows the same annual modulation as DAMA. Oops.
Now, let’s just say upfront that the CoGeNT results, while intriguing, nonetheless don’t add up (yet) to a bona fide discovery: it’s one of those pesky less-than-three-sigma results — 2.8 sigma, in this case — hovering right on the threshold of discovery without crossing it. (Five sigma is the “gold standard” for discovery.) All Collar would say is that his team is “making all the data available to others so they can make their own interpretation.” But as Sean Carroll points out over at Cosmic Variance:
For those who haven’t been following this story, dark matter likely makes up around 23% of all matter in the universe. But scientists thus far have not been able to observe it directly, because it interacts so weakly with ordinary matter; we only infer its existence from detecting their gravitational fields.
A physicist named Fritz Zwicky first noticed this phenomenon in 1933 when he concluded that galaxies in the Coma cluster were moving so quickly that they should be able to escape from the cluster if visible mass was the only thing contributing to the cluster’s gravitational pull. Since the cluster hadn’t flown apart, he proposed the existence of “dark matter” to account for the observational data.
In the 1960s, Vera Rubin and Kent Ford confirmed Zwicky’s theory when their spectral analysis revealed that the outer stars in selected spiral galaxies were orbiting just as quickly as those at the center. The visible matter wasn’t sufficient to account for this; the spiral galaxy should be flying apart. Clearly, there had to be some kind of hidden “dark” mass adding to the galaxy’s gravitational influence.
Physicists have been trying to directly observe dark matter ever since. What are they looking for? Well, they’re not 100 percent sure, frankly, but they have some ideas. The leading candidate for dark matter is a Weakly Interacting Massive Particle (WIMP). But they also need to know where to look, in terms of range of mass. The more you can narrow the target range, the better your chances of detecting a WIMP. Most theorists seem to favor a “heavy WIMP” model — predicting a particle with a mass of around 100 GeV — while a few others have staked out a claim in favor of light WIMPs, with a mass of 7 or 8 GeV.
Each camp can cite hints of experimental evidence in its favor, but the issue is far from resolved. (Bear with me here, there are a lot of acronyms.) On the “light” side of the debate, you’ve got results from the Fermi Gamma-ray Space Telescope and DAMA, in which sodium iodide detectors are buried deep underground in the Gran Sasso mountains, emitting flashes of light (Cerenkov radiation) at those rare moments when dark matter particles collide with the detector material.
The strategy with DAMA is not to try to pick out individual dark matter signals from all that background noise, but instead to have tons of candidate events and look for slight changes in the number of observed events as the Earth orbits around the sun.
As Carroll explains, “Dark matter is like an atmosphere through which we are moving; when we’re moving into a headwind, the rate of interactions should be slightly higher than when our relative speed through the ambient dark matter is smaller.” The problem is that other experiments, employing complementary strategies, can’t replicate DAMA’s findings, making it more likely that the observed fluctuation isn’t due to dark matter.
On the “heavy” side of the debate, you have the CDMS and XENON100 collaborations. XENON100 uses 100 kg of liquid xenon deep underground in the same Gran Sasso mountains. Xenon is a very heavy element, and thus has a larger cross section, which determines the number of likely collisions (larger is better). XENON scientists say their results — or lack thereof, since they don’t see evidence for “light” WIMPs — rule out the light WIMP scenario.
Adding weight to XENON’s claims is the Cryogenic Dark Matter Search II (CDMSII), which uses very pure crystals of germanium and silicon buried in a deep mine in Soudan, Minnesota. Like XENON, CDMS opts for sensitivity, building an ultra-quiet experiment deep underground to shield the detectors from interfering factors like cosmic rays.
As Valerie Jamieson writes in New Scientist, “Both these experiments are so sensitive that they should have seen dark matter if the DAMA result is due to WIMPs.”
Enter CoGeNT. There have been earlier hints of a dark matter signal, also implying a light WIMP of about 10 GeV. And CoGeNT uses the same detector material as CDMS, so the latter’s null results were especially damning to CoGeNT’s tentative evidence for light WIMPs. But now we have a new twist in the tale: the latest CoGeNT data does indeed show an increase in collisions at certain times of the year, consistent with DAMA’s findings. This is the first time that’s happened — and Collar was not expecting it.
As for the latest XENON100 experiment that searched the same range and didn’t find dark matter — well, Collar thinks that perhaps that experiment covers less range than its scientists think it does. Dark matter could be lurking in one of the few remaining gaps in the “light WIMP” mass range.
Dark matter detection is tricky business. Physicists are going to be debating how best to interpret all these conflicting results for quite some time. But at this stage of the game, things are starting to look more promising for those in the “light” WIMP camp.
Image (top): Computer simulation of large scale structure. Credit: Science Magazine