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Gold nuclei hit by particles would cut genes

Dark-matter detector using DNA conceived

The Washington Post

That wonder molecule of life on Earth, DNA, is being enlisted in the search for an exotic species dwelling in the cosmos: dark matter.

As far back as the 1930s, astronomers watching distant galaxies saw that something was missing. There were not enough stars to account for the strength of gravity needed to whirl galaxies so quickly or smash them together so swiftly.

Something else must surround and suffuse every galaxy, some kind of gravitational glue.

Cosmologists dubbed it “dark matter,” as it sheds no light. They say it far outweighs all the ordinary matter — stars and planets — that they can account for.

The leading candidate for this mystery substance is subatomic particles called weakly interacting massive particles (WIMPs). They can’t be seen, but they should be nearly everywhere, at least in our galactic neighborhood.

If they exist, then every once in a great while, a zooming WIMP should by chance smack into the nucleus of an atom.

For two decades, physicists have built detectors crammed with dense crystals and other heavy materials to try to catch WIMPs in this act. The results have been largely equivocal, although hints have appeared.

Proposals for the next generation of dark-matter detectors run into the tens or hundreds of millions of dollars. One such project would require an empty mine filled with a cubic kilometer of gas.

Now, though, a group of big-name theoretical physicists and biologists has proposed a radical new type of detector that dangles DNA as dark-matter bait. The size of a coffee table, it would be much less expensive than other proposed detectors, they say.

Call it the ultimate mashup of biology and cosmology.

“For the very first time, an important problem in physics can be solved by techniques from another science,” said Andrzej Drukier, the physicist-turned-biologist who struck on the idea.

Drukier proposed a DNA detector in a 2010 talk at UCLA. Soon after, Katherine Freese, a University of Michigan theoretical physicist, joined him. The pair are well placed to leapfrog the hunt for dark matter. In 1986, they laid out the theoretical rationale that led to the current generation of detectors.

This spring, Drukier and Freese drove to the San Diego home of Charles Cantor, a pioneer of the Human Genome Project who built a DNA technology company, Sequenom. They sat around his pool overlooking the Pacific, batting around ideas. A challenge immediately appeared.

“When you’re trying to think across such a vast range of disciplines, finding a common language is difficult,” Cantor said. “So we scribbled lots of pictures on pieces of paper — pictures instead of equations.”

From those scratchings, a rudimentary design emerged, a cube about a meter on a side.

In it, thousands of strands of DNA hang from sheets of gold, like row upon row of beaded curtains. When a WIMP zooms in, it may hit a gold nucleus, knocking it free and sending it through the DNA curtains, slicing them. (Gold, with its heavy nucleus, makes a good choice for a material to rip through the DNA.)

The severed DNA will drop to the bottom of the detector and be collected. Standard DNA-reading machines then will reconstruct the path of the WIMP through the curtains.

By tracing the direction of the WIMP, the DNA detector offers crucial information that current detectors cannot provide: It tells you where in the sky the WIMP came from.

That is essential for testing dark-matter theories. As our solar system circles the center of the Milky Way, it currently is heading toward the constellation Cygnus. The dark matter in our galaxy, which should be more or less stationary, should appear to be streaming in from that direction.

As Freese said, “We’re moving into this headwind of WIMPs.” The DNA detector should be able to sense this wind like a hand stuck out the window of a moving car.

Freese said that if the detector works, finding evidence of just 30 WIMPs will be enough to prove that these elusive particles exist.

Early on, Drukier passed the idea to other top DNA scientists, including Takeshi Sano of Japan and Harvard’s George Church.

“I said, ‘I have this crazy idea; kill it,’ ” Drukier said. “They tried, but they couldn’t.”

Instead, both signed on as collaborators.

“This is exactly my kind of challenge,” Church said. In October, Drukier spent a week in Church’s lab “hammering out details.”

In June, the collaborators unveiled the concept in the arXiv, an online repository of physics papers. The response, Freese said, was immediate and positive. Invitations for talks in Paris and New York arrived; recently, she peddled the idea to a standing-room-only crowd at CERN, the European particle physics laboratory.

Optimistically, Freese said, they can build the device within five years for perhaps $100,000. She recently won a small grant from the University of Michigan for pilot studies.

“We can do this,” Cantor said. “It all comes down to cost.”

Church is similarly optimistic. He said most of the DNA technology needed for the project already exists. Soon, he said, a gram of manufactured DNA will cost less than a gram of gold. Freese estimates they will need a kilogram of each.

First, Freese will test a crucial aspect of the concept: How, exactly, will the nucleus from a gold atom slice through strands of DNA? What pattern might it create? She expects to start such tests within a few months at a small particle accelerator that her university rents for $50 an hour. By the standards of most physics experiments, she said, “that’s like free.”