Friday, April 5, 2013

Dark matter MRI could boost hunt for hidden particle

Every underdog has its day. Using the same underlying physics as MRI scanners, a new experiment could boost the search for one of the humbler dark matter candidates: the axion.

Cosmologists think dark matter makes up almost 80 per cent of the universe's matter, but no one knows what it is made of. Most hunts for dark matter focus on detecting individual, highly elusive particles known as weakly interacting massive particles (WIMPs).

For instance, a dark-matter-hunting telescope perched on the International Space Station has been looking for signs of WIMPs smashing into each other and decaying into electrons and their antimatter counterparts, positrons.

This interaction should change the ratio of electrons to positrons compared to background levels. First results from the experiment, called the Alpha Magnetic Spectrometer, show tantalising hints of this process, but it is still too early for any experiment to say with confidence that WIMPs exist.

The runner-up candidate for the dark matter particle is the axion, which should behave more like a background fluid. Trouble is, it is even harder to find.

Dark matter magnets

Peter Graham of Stanford University in California and colleagues presented a possible solution at the SnowDARK workshop in Utah on 24 March. "This is one of the first new ideas in axion detection in a long time," he says.

Theory has it that axion fluid should separate the charge inside protons and neutrons, giving them what is called an electric dipole moment, so that they have positive and negative poles, like magnets.

If axions exist, applying a magnetic field to a lump of lead would align the dipoles in a specific direction, and applying a perpendicular electric field would then set the dipoles spinning like wobbly tops. That spin generates a secondary magnetic field that the experiment could detect ? akin to what happens in an MRI machine.

Douglas Finkbeiner of Harvard University is excited about the idea: "They have proposed a clever and cost-effective way to search for axions, and I hope they can make it work."

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