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New clues into the existence of dark matter

The newly installed Alpha Magnetic Spectrometer-2 is visible at center left in this photo of the International Space Station's starboard truss.


For Chris Hadfield and other astronauts aboard the International Space Station, the high-energy particles that come flying at them from deep space and pass through their equipment and their bodies like subatomic bullets are a hazard and a menace.

For scientists, they are cosmic gold.

Now researchers sifting through that gold – gathered up by a unique cosmic-ray detector bolted to the station – say they've confirmed the existence of a strange new source of particle radiation in the galaxy.

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The identity of the source remains unknown, but the most tantalizing possibility is that the detector has delivered concrete evidence of dark matter, a ghostly substance that is believed to be the predominant material of the universe even though it has never been seen.

If dark matter has in fact been discovered, the find will open the door to an entirely new domain of physics beyond the Standard Model, the theory that predicts and explains all known forms of matter, including the recently discovered Higgs boson.

"This is a very happy day for all of us," said Samuel Ting, the Massachusetts Institute of Technology physicist who leads the experiment, during a NASA press briefing on Wednesday.

Dr. Ting and his colleagues weathered years of criticism while trying to get their $2-billion experiment, the Alpha Magnetic Spectrometer (AMS), off the ground. In 2005, the device was taken off the flight manifest for the space shuttle after Dr. Ting gave a poor presentation to NASA's administrator, he said. It was eventually reinstated in time to catch the penultimate shuttle flight and arrive at the station in May, 2011.

Since then, AMS has been recording incoming cosmic rays – high-energy particles that originate beyond our solar system, mostly from such exotic sources as exploding stars and black holes.

The new results, to be published this month in the journal Physical Review Letters, focus on a rare type of positively charged particle known as the positron. As AMS looks toward higher and higher energies it detects an increasing number of positrons, contrary to expectation. A European-Russian satellite called PAMELA reported similar findings in 2009, but AMS is far more precise, leaving no doubt that the signal is real.

Scientists widely agree there are only a couple of plausible explanations for the excess positrons. One is that they are somehow being created and accelerated by a rapidly spinning dead star, called a pulsar.

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More exciting is the prospect that invisible dark-matter particles adrift in space are colliding with each other and annihilating in a burst of energy that generates positrons.

"We are more ignorant about dark matter so we should be careful about jumping to conclusions," said Neal Weiner, a theoretical physicist at New York University who is not part of the AMS team. "At the same time, there are reasonable models of dark matter that would give this."

Dr. Ting said he expects AMS will distinguish between the two explanations "in a year or two," after more data are gathered at higher energies.

"I think with AMS there is no question that we are going to solve this problem," he said.

Physicists are eager for any clue that would reveal the nature of dark matter, which in turn could shed light on the origin and history of the universe. Although dark matter emits no light and does not interact easily with ordinary matter, its existence has been inferred through its gravitational pull on stars and galaxies. Europe's Planck satellite recently showed that the dark matter present in the universe must outweigh ordinary matter by more than a factor of five.

The AMS result will have a bearing on experiments now in development to detect dark matter passing through Earth. One Canadian-led experiment, called DEAP-3600, is scheduled to begin in 2014. It will operate more than two kilometres below ground at the Sudbury Neutrino Observatory, which is well-shielded from surface effects that can mask a dark-matter signal.

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If AMS is seeing dark matter indirectly, "it makes the upcoming search for the direct detection of dark matter in underground experiments very exciting," says Mark Chen, a physicist at Queen's University in Kingston, and a member of the DEAP-3600 team.

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