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No Signs of Dark Matter Yet from the World's Deepest Underground Lab

Fear not, the search is hardly over.
Image: NASA/ESA

China's Jinping Underground Laboratory is the deepest underground lab in the world. Accessible only through a 17-kilometer tunnel, the facility sits under 2.4 kilometers of solid rock. It is a quiet place, at least with regards to the steady bombardment of cosmic rays experienced at Earth's surface. Jinping is perfect for hunting dark matter, in other words.

The lab is home to the PandaX dark matter detection experiment. The detection technology used in the lab is based on a cube full of 100 kilograms of liquid xenon, making it similar to detectors currently at work under a mountain in Italy (the XENON100 experiment) and deep in a North Dakota mine (The LUX experiment). Given that dark matter accounts for the vast majority of matter in the universe, it would really be hard to say that we're putting too much effort into actually detecting the stuff.

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Something of a late-comer to the hunt/race for a dark matter direct detection, PandaX is just now releasing its first batch of results, published in the journal SCIENCE CHINA Physics, Mechanics & Astronomy.

As you've already gathered, those results are negative for dark matter events. This doesn't mean dark matter doesn't exist by any stretch, but it does mean that it doesn't exist within the range of particle masses explored by the experiment. It's like looking for something in a dark room with a candle—it's impossible to see the whole room at once, so the searcher is forced to go from corner to corner, etc.

The PandaX results appear to align well with results from the LUX experiment reported in 2013, with both establishing similar lower-limits to the expected masses of dark matter particles. Not only does that help narrow the search generally, it also once again calls into question earlier detection-signals of very low-mass dark matter particles, such as those reported by researchers at the SuperCDMS experiment, itself housed nearly a kilometer deep in a defunct mine in northern Minnesota.

The quietest places on Earth

The reason for all of this subterranean science is the need to eliminate as much as possible the interfering effects of background radiation of the sort that's normally bombing each and every one of us at any given time. The more distance lying between a detector and the surface, the less background. Dark matter, meanwhile, should be relatively unaffected by the rock barrier. That's just dark matter.

The particular variety of dark matter being hunted are known as WiMPs, or weakly interacting massive particles. Once WiMP particles arrive at these various underground detectors undeterred by many thousands of meters of rock, they meet the liquid xenon.

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This supercold, superdense pool filters out particles (rays) that have lower energies, leaving only the most energetic particles to hit the center of the xenon tank. These highly energized particles are much easier to analyze as data and, thus, it's much easier for physicists to determine what sort of particle has collided with a xenon particle.

These collisions between incoming rays and xenon nuclei have the highly desirable effect of creating tiny bits of light, which is registered and amplified via a clever scheme involving photodetectors. A dark matter detection hides within these light signals. The xenon detector behind the PandaX experiment is quiet enough such that of four million "raw events," only 46 particles actually made it to the center of the tank.

PandaX is built to examine both high- and low-mass ranges. The next phase of the experiment will boost the tank capacity to nearly a ton of liquid xenon. This means that the particles making it that much farther into the central target of the tank must be extremely energetic. Meanwhile, the LUX detector, which already boasts a much higher xenon volume (almost 400 kilograms of the stuff) and can thus search for higher energy particles as is, will continue collecting data until next year.

These "quiet" detectors are an interesting contrast to the detector found on the International Space Station, the Alpha Magnetic Spectrometer. The AMS specializes in noise, parsing the collisions that result from the continuous barrage of cosmic rays against Earth's upper atmosphere. By statistically chewing through this din, the AMS should be able to register any "extra" collision events.

So, don't be too discouraged by negative results from the xenon detectors: the AMS appears to be hot on the trail of a mysterious cosmic ray interloper, possibly dark matter.