- Associated Press - Monday, June 15, 2015

LEAD, S.D. (AP) - Nearly two years have passed since scientists with the Sanford Underground Research Facility announced the Large Underground Xenon (LUX) experiment nestled one mile below the earth’s surface in the former Homestake gold mine was the most sensitive dark matter detector in the world. A lot has happened since then, just not on a spectrum that registers much fanfare outside of the science community.

What’s happening is a lot of data collection. The world-class dark matter detector is currently about a third of the way through a 300-day data collection run that began in October 2014, the Black Hills Pioneer (https://bit.ly/1TbXa6F ) reported. South Dakota Science and Technology Authority (SDSTA) Executive Director Mike Headley said LUX scientists expect to complete their data review sometime in 2016.

What exactly is team LUX looking for?

The simple answer is dark matter, the yet-to-be-detected stuff that the vast majority of the scientific community believes works with gravity to hold all matter in the universe together.

Based solely on observable particle physics, all the galaxies in the universe should’ve torn to bits ages ago, as they’re rotating fast enough to overcome the gravitational force created by the observable matter they’re comprised of. This led scientists to the notion that something else must be working with or in addition to gravity to keep everything together. That mysterious substance is currently undetected and does not interact with the universe’s electromagnetic force, meaning it doesn’t emit, reflect, or absorb light, hence “dark matter.” And while we have yet to find dark matter, scientists have calculated roughly how much of this stuff must exist in the known universe in order to keep everything from flying apart. Roughly 27 percent of the universe seems to be comprised of dark matter.

There are several theories on what dark matter may be, the prevailing one holds that it’s made up of so-called Weakly Interacting Massive Particles, or “WIMPs,” which are thought to be all around us in massive quantities but practically invisible - electromagnetically neutral loners that very rarely, if ever, collide with other particles.

But what if WIMPs do collide with other particles on occasion? This is where LUX comes in.

LUX is a 6-foot-tall titanium tank filled with 815 pounds of liquid xenon and an array of photomultiplier tubes. The detector is housed in a two-story water tank filled with ultra-purified water. That tank sits cozy one mile below the surface of the earth at the Sanford Lab’s 4850 level.

Xenon particles generate a burst of light when other particles collide with them. This is what the photomultiplier tubes are there to capture. The detector is squirrelled away so deeply underground in order to limit the amount of particle interactions occurring at any moment. That number would be overwhelming on the surface, where cosmic rays constantly rain down from the skies and background radiation levels are much higher than they are beneath 4,850 feet of shielding rock.

LUX’s photomultiplier tubes catch the light flashes emitted in the detector and log data about the energy generated by these collisions. Scientists then sort through the subsequent gargantuan leagues of data in search of the unique energy signature of a WIMP. Finding one brings us that much closer to finally defining dark matter, which is a very big deal.

Out of all the dark matter detectors currently operating in the world, LUX is still the most sensitive. This means scientists working in a repurposed gold mine in South Dakota have a greater chance at discovering dark matter than anyone else on the planet.

But what happens if LUX doesn’t find any WIMPs? Simple: we build a bigger LUX.

LUX 2.0 is already in the workings. Construction is set to begin on the LUX-ZEPLIN experiment, or “LZ” for short, after LUX shuts down for good sometime in 2016. LZ will be twice as sensitive as LUX, its detector holding some 10 metric tons of liquid xenon compared to LUX’s one-third of a ton. Maybe the muscle car guys are right: there’s no replacement for displacement.

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Information from: Black Hills Pioneer, https://www.bhpioneer.com

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