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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....
WIMPs would be moving in random directions in this halo, at velocities similar to the sun’s. “You find yourself in a situation similar to a car moving through a cloud of gnats,” Collar explained. “The faster the car goes, the more gnats will hit the front windshield.”
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). The Earth orbits the sun at 15 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.
Collar and his colleagues have calculated the probability that their finding is a fluke to be five-tenths of a percent, or 2.8 sigma in particle physics parlance.
“It’s not an exact science yet, unfortunately,” Collar said. “But with the information we have, the usual set of assumptions that we make about the halo and these particles, their behavior in this halo, things seem to be what you would expect.”
Other dark-matter experiments, including Xenon100, have not detected the seasonal signal that CoGeNT and DAMA have reported.