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Space is Only Noise in the Underground Dark Matter Detector

For all of the pop culture, mainstream buzzing devoted to the Higgs boson, it seems almost bizarre that at least some amount of comparable attention isn't devoted to dark matter and dark energy. Is that the price tag that comes with high energy...

For all of the pop culture, mainstream buzzing devoted to the Higgs boson, it seems almost bizarre that at least some amount of comparable attention isn’t devoted to dark matter and dark energy. Is that the price tag that comes with high energy collider-based research? The smashing of things together? Those much-maligned, yet much-doted-upon words “god particle”? Meanwhile, dark matter is a mystery potentially beyond the Higgs boson and its Standard Model home. Dark matter doesn’t even fit on our current Standard Model, the rulebook describing the universe’s fundamental forces and how they interact. Even cosmologists don’t really know, in the words of William Shatner, what the fuck it is. Yet the stuff makes up the vast majority of matter in the universe: estimates have that our normal “light” universe is only about five percent of the entire universe’s contents, while dark matter fills in 25 percent, and dark energy 75 percent. And yet, our search for the stuff is only just beginning. results from a key dark matter detection experiment at the Laboratori Nazionali del Gran Sasso in Italy are in, and looks like we’re still yet to even directly detect it.

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The particle we’re looking for at Gran Sasso (and elsewhere) is what’s known as a WIMP, or Weakly Interacting Massive Particle, which is actually more of a placeholder for whatever specific particle winds up the dark matter solution. So far, there are three possibilities: neutrinos, axions, and neutrilinos, with the latter two being the more likely candidates. Those two also happen to be hopes for fixing gaps in the Standard Model, which we now know is incomplete. So, if and when we find a WiMP, we’ll also have a better Standard Model. WiMPs work out quite well for both particle physicists and astronomers, if the particles exist.

We know dark matter in general exists, however, in some form because of its gravitational effects on galaxies. In a sense, we’ve already seen dark matter, but only through its effects. Which is hardly enough. The Gran Sasso experiment, called XENON100, is situated about 120 km from Rome and deep beneath a mountain, so it’s shielded from other cosmic rays. It’s job is to record tons of data about particles and their movements in a big tank of super-cooled xenon, which is further shielded by two kinds of lead, plasic, copper, water, and a mile of rock (between the detector and the mountain’s surface). There is nonetheless still background activity to sort through.

Data taken during 13 months of XENON100’s operation at Gran Sasso reveals no hints of WiMPs, according to a paper posted on the arXiv pre-print server (and submitted to the Physical Review Letters). There’s a bright side here, kind of, as it further limits the mass range where the particles could be found. A limited range means better targeted searches; we know where WiMPs aren’t so we don’t have to waste time looking there. A release from Gran Sasso explains, “The new data improve the bounds to 2.0 × 10^-45 cm^2 for elastic interaction of a WIMP mass of 50 GeV, which is another factor of 3.5, cutting already significantly into the expected WIMP parameter region.”

The search isn’t over at Gran Sasso by any means. The XENON100 will keep running until the still-under-construction XENON1T comes online. Whereas the XENON100 uses 100 kg of xenon liquid as its potential dark matter crash pad, the new detector will use 2.2 tons of xenon, drastically increasing the volume for a collision and, thus, detection to happen. And if it doesn’t, scientists still have no shortage of ideas to explain that full quarter of the entire universe we can’t yet see.

Reach this writer at michaelb@motherboard.tv.

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