Thursday, August 15, 2013

Underground Andes Lab to Tackle Unresolved Dark Matter Mystery

High up in the Andes mountains, civil engineers are laying out plans to build an almost nine-mile-long tunnel connecting Chile and Argentina. Once completed, the Agua Negra tunnel will be the sixth longest underground road in the world. There will be points along the way where cars will be driving more than a mile below Earth's surface. It's going to be an amazing engineering achievement to build.

It's also going to be the perfect place for a new underground physics laboratory that could resolve one of the biggest outstanding mysteries of dark matter.

The three-chambered ANDES underground laboratory would split off from the subterranean roadway.
Image: Xavier Bertou  
South American scientists have been putting together a proposal to build a lab at the tunnel's deepest point. There, under more than 5,700 feet of natural radiation shielding, the Agua Negra Deep Experiment Site, aka ANDES, will be filled with sensitive detectors looking for ghostly neutrinos and dark matter particles.

"The underground laboratory is basically a place with zero-level of radiation. So any measurement where radiation is an issue can be done in such a lab," said Xavier Bertou, a researcher at Argentina's Bariloche Atomic Centre and member of the ANDES planning team. "This includes basic science, dark matter and neutrino physics, but there is a lot of applied science based on measuring low radioactive levels."

The lab would offer ideal experiment space for dark matter detectors.
 Image: Xavier Bertou
Scientists are particularly excited about what ANDES might tell us about dark matter. Decades of inexplicable astronomical observations have led most physicists to conclude that most of the matter in the universe is invisible. Theorized particles called WIMPs, short for weakly interacting massive particles, are the leading candidates for what makes up this missing 85 percent of the universe. As the only planned dedicated dark matter detectors south of the Equator, ANDES is in a unique position to hunt for these elusive dark matter particles.

Right now one of the most tantalizing and controversial results in the hunt for WIMPs have been from the DAMA/LIBRA detectors in Gran Sasso, Italy. Since 2008, scientists working on the experiment claim to have seen a clear seasonal signal of dark matter particles. The results are controversial, because the detectors aren't very good at differentiating potential WIMPs from other background signals.

During summer scientists see more particles hitting their detectors, which then drops off during wintertime. The idea is that our galaxy is effused with a "halo" of dark matter, and as the solar system moves through this cloud, it's feeling a kind of wind of invisible matter. In July the orbiting Earth is moving directly into the wind, hitting more particles, while in January it's traveling with the wind, hitting fewer.

"The question is therefore whether the modulation observed for example by DAMA/LIBRA is produced by the Dark Matter wind or by some atmospheric effect," Bertou said. "If one has a detector in the southern hemisphere, the Dark Matter wind should produce the same modulation, while the atmospheric explanation would give an opposite modulation as the seasons are inverted. Therefore, having a DAMA/LIBRA or similar detector in the south would allow to disentangle these two effects."
A pit 100 feet across and 100 feet deep would host
a large liquid neutrino detector.
Image: Xavier Bertou
ANDES would host a number of other experiments unique to the hemisphere. Right now Antarctica's IceCube experiment is the only cosmic-neutrino detector set up south of the Equator; ANDES would host the second. This detector could not only be used for looking at these particles coming from the southern sky, but also to look for neutrinos coming from Earth itself, hinting at how some of the Earth's core is heated by radioactive decays.

Building the lab along with the underground roadway is a good way to economize it's construction. Other road tunnels around the world have incorporated underground into their designs including Italy's Gran Sasso lab where DAMA/LIBRA is located.

Both Chile and Argentina have expressed strong official interest in the project so when the final plan for the Agua Negra tunnel is chosen by the end of this year, it should include the laboratory as well. At the same time, Brazil and Mexico have joined an international consortium of Latin American countries planning to run the facility once its completed. Construction on the Agua Negra tunnel should start in 2014 and if all goes according to plan, the lab could open in 2021.

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