Oh, dark matter,February 2011. you saucy tease! Just when we think were zeroing in on finally figuring you out, you start flirting with some other experimental apparatus, just to keep everyone guessing.
gold detectoDark Matter Debate Heats Up… Again,Thats right,cheap wow gold. short sellin. 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 Chicagos Juan Collar, head of the CoGeNT experiment, at a meeting of the American Physical Society in Anaheim, California.
ANALYSIS: How Low Can a Dark Matter Halo Go?
CoGeNTs 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 Italys DAMA experiment, which detected tiny periodic fluctuations in the rate of events several years ago over a year-long time scale.
DAMA scientists think its dark matter. Other physicists arent 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 CoGeNTs most recent run shows the same annual modulation as DAMA. Oops.
Now, lets just say upfront that the CoGeNT results, while intriguing, nonetheless dont add up (yet) to a bona fide discovery: its 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 all the data available to others so they can make their own interpretation. But as Sean Carroll points out over at Cosmic Variance:
[I]ts the first attempt to check DAMA by looking for an annual modulation signal, and the result matches the phase of DAMAs oscillation, and is claimed to be consistent with its amplitude (the experiments use different materials, so its hard to do a direct comparison). Also, of course, because the team was looking to bury DAMA, not to praise it: We tried like everyone else to shut down DAMA, but what happened was slightly different.
For those who havent been following this story, dark matter likely makes up around 23% of all matter in the universe. But scientists thus r 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 clusters gravitational pull. Since the cluster hadnt flown apart, he proposed the existence of dark matter to account for the observational data.
In the 1960s, Vera Rubin and Kent Ford confirmed Zwickys 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 wasnt 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 galaxys gravitational influence.
Physicists have been trying to directly observe dark matter ever since. What are they looking for? Well, theyre 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 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 vor a heavy WIMP model — predicting a particle with a mass of around 100 GeV — while a few others have staked out a claim in vor of light WIMPs, with a mass of 7 or 8 GeV.
ANALYSIS: Where is Dark Matter Hiding?
Each camp can cite hints of experimental evidence in its vor,May 2011. but the issue is r from resolved. (Bear with me here, there are a of acronyms.) On the light side of the debate runescape money, youve 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,← Older posts. 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 were 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, cant replicate DAMAs findings, it more likely that the observed fluctuation isnt 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 dont see evidence for light WIMPs — rule out the light WIMP scenario.
Adding weight to XENONs 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 ctors 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 latters null results were especially damning to CoGeNTs 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 DAMAs findings. This is the first time thats happened — and Collar was not expecting it.
As for the latest XENON100 experiment that searched the same range and didnt 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
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