Deep in the dark of what was once the largest and deepest gold mine in the United States, scientists with the Large Underground Xenon (LUX) Experiment are nearly ready to start their search for another type of dark — dark matter.
The rather ironically named LUX experiment ('lux' is a measure of light intensity) is part of the Sanford Underground Research Facility, which has been constructed in the former 'Homestake Mine' in Lead, South Dakota. The facility is specifically designed to house physics experiments that are so sensitive that, were they conducted at the surface, their results could be skewed by cosmic radiation. The first such experiment was done by nuclear chemist Ray Davis back in 1965, to test the 'solar neutrino problem' — that we were detecting far fewer solar neutrinos than were predicted by our standard idea of how stars worked — and his work earned him a share of the 2002 Nobel Prize for Physics. The Homestead Mine was closed in 2002, and was selected by the National Science Foundation in 2007 to house the new research facility.
LUX is searching for something that cannot be seen directly by telescopes because it emits no light and no radiation, at least none that we can detect. We can only 'see' dark matter in two ways (at the moment): We can see the effect its gravity has on objects close to it (because ALL matter in the universe exerts some gravitational force, however tiny, on everything else in the universe), or we can know its there by how its gravity increases the intensity of light or radiation that passes near it — what's known as 'gravitational lensing', where space itself is warped by a massive object and acts like a telescope lens. Based on the measurements of the rotation and motion of galaxies (including our own), and thus the mass of the visible universe, and on measurements of the mass of all objects that emit some sort of radiation we can detect, it is estimated that around 84% of all the matter in the universe is dark matter.
Why is detecting dark matter important? Why spend the money on this? Knowing how much dark matter there actually is and being able to know where it is will help us to answer some of the fundamental questions about how the universe formed, why it has the structure it does and what the ultimate fate of the universe will be. It could possibly lead to new discoveries we haven't even considered yet.
"We might well uncover something fantastic," said LUX principle investigator Harry Nelson, a physics professor at the University of California, Santa Barbara. "One thing about our field is that it's kind of brutal in that we know it's expensive and we work hard to only do experiments that are really important."
Detecting dark matter is extremely difficult. The scientists with LUX have set up a large vacuum thermos that holds 350 kg of cooled liquid xenon at a temperature of -107 degrees C, and they are looking for collisions between the xenon atoms and dark matter particles called WIMPs (weakly interacting massive particles). The xenon will constantly circulate through filters that will remove any contaminating particles and avoid false alarms that have plagued other detection experiments.
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"Basically the detector works like a turnstile," said Jeremy Mock, a physicist who has been working on the LUX experiment for the past five years. "It's designed to detect any particle that moves through it, and we're looking for WIMPs."
Current projections are that the detector could start collecting data as early as February, and would be fully operational by March, when it can claim its rightful place as the world's most sensitive dark matter detector.