New Data Still Have Scientists in Dark Over Dark Matter
ScienceDaily (June 8, 2011) — A dark-matter experiment deep in the Soudan mine of Minnesota now has detected a seasonal signal variation similar to one an Italian experiment has been reporting for more than a decade.
Juan Collar, associate professor in physics at the University of Chicago, holds one of the early germanium detector prototypes similar to the one being used in the Coherent Germanium Neutrino Technology (CoGeNT) experiment, situated nearly half a mile deep in the Soudan Mine in Minnesota. Such underground locations help screen out false dark-matter signals from other natural sources of radiation. Detectors of this kind are used because of their sensitivity to weak levels of radiation. (Credit: CoGeNT Collaboration)
The new seasonal variation, recorded by the Coherent Germanium Neutrino Technology (CoGeNT) experiment, is exactly what theoreticians had predicted if dark matter turned out to be what physicists call Weakly Interacting Massive Particles (WIMPs).
"We cannot call this a WIMP signal. It's just what you might expect from it," said Juan Collar, associate professor in physics at the University of Chicago. Collar and John Orrell of Pacific Northwest National Laboratory, who lead the CoGeNT collaboration, are submitting their results in two papers to Physical Review Letters.
WIMPS might have caused the signal variation, but it also might be a random fluctuation, a false reading sparked by the experimental apparatus itself or even some exotic new phenomenon in atomic physics, Collar said.
Dark matter accounts for nearly 90 percent of all matter in the universe, yet its identity remains one of the biggest mysteries of modern science. Although dark matter is invisible to telescopes, astronomers know it is there from the gravitational influence it exerts over galaxies.
Theorists had predicted that dark matter experiments would detect an annual modulation because of the relative motion of Earth and the sun with respect to the plane of the Milky Way galaxy.
The sun moves in the plane of the galaxy on the outskirts of one of its spiral arms at a speed of 220 kilometers per second (136 miles per second). Earth orbits the sun at 30 kilometers per second (18.5 miles per second). During winter, Earth moves in roughly the opposite direction of the sun's movement through the galaxy, but during summer, their motion becomes nearly aligned in the same direction. This alignment increases Earth's net velocity through a galactic halo of dark matter particles, whose existence scientists have inferred from numerous astronomical observations.
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Journal References:
1. C.E. Aalseth, P.S. Barbeau, J. Colaresi, J.I. Collar, J. Diaz Leon, J.E. Fast, N. Fields, T.W. Hossbach, M.E. Keillor, J.D. Kephart, A. Knecht, M.G. Marino, H.S. Miley, M.L. Miller, J.L. Orrell, D.C. Radford, J.F. Wilkerson, K.M. Yocum. Search for an Annual Modulation in a P-type Point Contact Germanium Dark Matter Detector. arXiv.org, 2011; [link]
2. J.I. Collar. A Realistic Assessment of the Sensitivity of XENON10 and XENON100 to Light-Mass WIMPs.arXiv.org, 2011; [link]
