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Looking for the Darkness in Ice

Buried underneath a mile of solid ice at the bottom of the world, two instruments are at the forefront of a new effort to corroborate or refute one of the most controversial scientific results in the ongoing search for dark matter. 
One of the DM-Ice17 detectors being lowered into the ice at the South Pole in 2010.
Image: The University of Wisconsin-Madison

The University of Wisconsin's DM-Ice17 experiment wrapped up two years of data collection at the South Pole in its hunt for the universe's missing mass. The two prototype detectors, immersed under a mile and a half of solid ice, are looking for the faintest pinpricks of light from a single particle of dark matter colliding with an atom. The two detectors finished their run in June and are the precursors to a future experiment that will be able to see if the Earth is passing through a cloud of invisible dark matter. It's the first experiment to look for dark matter south of the equator.

"This is a prototype of the full scale," said Reina Maruyama, a professor at Yale and the spokesperson of the DM-Ice collaboration. She added that the test devices aren't big enough for a definitive measurement and the background noise is too loud, but it shows that "you can operate detectors like this remotely, buried in the ice at the South Pole, and that the ice is quiet enough to do this."

Dark Matter

A DM-Ice17 detector outside of its protective sheath.
Its sodium iodide crystal is in the middle behind a
layer of copper shielding, with detectors and photo-
multiplier tubes above and below.
Image: The University of Wisconsin-Madison
Astronomers studying the way galaxies rotate have long concluded that there's just too much stuff out in the universe to be accounted by ordinary stars, planets and dust alone. There has to be something else out there, a lot of something. This dark matter likely makes up a majority of the matter in our universe, about 85 percent of it. What's more, because we can't see it, it has to be totally invisible to all wavelengths of light.

No one really knows for sure what this dark matter is made of, but it's most likely some enigmatic new particle that scientists have dubbed WIMPs, short for Weakly Interacting Massive Particles. Seeing these unseeable particles is no easy feat, and there are experiments all over the world trying different strategies to find it.

The DM-Ice collaboration is based off a controversial series of Italian experiments called DAMA. They're controversial because the team claims to have found dark matter, even though almost nobody else has been able to detect it at all.

"The whole project of DM-Ice is trying to test DAMA's claim that they have observed dark matter," Maruyama said. "We're going to use the same detector materials."

Even more frustratingly, another, similar experiment call COGENT has seen some hints, but nothing remotely conclusive.

"None of the others have signals as strong as DAMA," Maruyama said. "There are other experiments that do not see that... The current picture is confusing."

Annual Modulations

DAMA, COGENT and DM-Ice are all looking for evidence that Earth is ploughing through a dense cloud of invisible dark matter that permeates throughout the galaxy. The way these experiments are designed, they can't identify very much about the individual particles that pass through their detectors, but scientists can look for a telltale pattern that says "here be dark matter."

When the universe first formed 13 billion years ago, dark matter coalesced into thick clouds throughout the universe. Normal matter started getting pulled in and eventually formed into primordial galaxies, which started rotating within the dark matter clumps.

Today, mature galaxies still rotate within these clouds of WIMPS, their constituent stars surging headfirst into this invisible miasma. Anyone on the star would experience a headwind of dark matter as it plows through.

Of course, we don't live on the surface of the Sun (that's impossible for lots of reasons); We live on the planet Earth which orbits around the Sun. The plus side is that should lead to some unmistakable signals in a specially built detector. When the Earth's orbit swings the planet into the wind, it should collide with more dark matter while when it's orbital path swings it with the wind, that collision rate should drop off. This should lead to a predictable annual rise and fall in detections.

Designing a detector that can see this is tough, but there is one trick that can give scientists a glimpse. DAMA built one in 1997 and have claimed to see this annual modulation over seven years. DM-Ice is trying to see the same thing, but from the South Pole.

"That signal should be the same anywhere on Earth," Maruyama said. "Many things can vary as the seasons do so we wanted to go to the southern hemisphere where the seasons are reversed in phase."

Even more confusingly, not all dark matter experiments agree. Again, COGENT has seen some evidence of the same kind of yearly modulation, but it's a lot weaker. Another underground detector, called LUX and located in South Dakota, recently announced their first results, and they didn't find anything at all.

If DM-Ice sees the same variation that DAMA does, then that could be a clue that they really are seeing dark matter. If the collision rates they're seeing are reversed, they're seeing something else, probably a signal from the Sun.

"DAMA's biggest hook is the fact that there is this annual modulation, with the phase, the peak, occurring at the right time of the year to be dark matter,"Maruyama said. "DAMA has observed it [but] people are still skeptical."

Seeing the Invisible

Dark matter is invisible because it doesn't interact with electromagnetic radiation. This means it also passes through ordinary matter almost totally unimpeded. However, on the rare occasion when a WIMP is perfectly lined up and strikes an atom's nucleus directly, it jiggles the nucleus and leaves behind a faint photon. That single dim photon is a signal we can see. Statistically, it's very rare for this to happen, but there are so many of them in a small space, that scientists can see when it happens in a specialized detector.

Scientists have built detectors like this. DM-Ice17 is one of them. So are DAMA and COGENT. DAMA and DM-Ice are built around a transparent sodium iodide crystal with sensitive light sensors on either end. They're sensitive enough to see the single photons left behind when a WIMP hits a nucleus.

A schematic of IceCube.
DAMA is nearly a mile under the Italian alps. DM-Ice17 is part of the giant IceCube neutrino detector array, buried under a mile and a half of ice at the South Pole. It's to block out interference from cosmic rays that would pollute the experiment's data.

"All we see form these detectors is light, so anything that can produce light in these detectors can show up as a signal," Maruyama said. "The South Pole, as it turns out, has a fantastic scientific infrastructure."

The data from DM-Ice17 is just a prototype of the final project. Its data is still too noisy, but it shows that their experiment fundamentally works. Maruyama and her team are still building the final detector that they hope to put in place within the next two years.

The DM-Ice team at the South Pole. From left to Right: Albrecht Karle, Klas Hultqvist, Kurt Woschnagg, Perry Sandstrom, Andy Arbuckle and Reina Maruyama
Image: The University of Wisconsin-Madison


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