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1 New LUX experiment: No dark matter in this corner on Fri Nov 22, 2013 11:46 pm


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The LUX detector.

Lawrence Berkeley Lab

Dark matter is perhaps the most frustrating substance we know. Invisible to all forms of light, we detect its presence through its gravitational influence alone. That's insufficient, though: physicists and astronomers alike would like to know what it is. Yet so far, we're better able to say what it isn't: it's not the stuff of normal matter (electrons, quarks) or other particles we know about, like neutrinos.
The process of figuring out what dark matter isn't received a major push today with the announcement of the first three months of data from the Large Underground Xenon (LUX) detector. Unfortunately, LUX failed to find any dark matter, ruling out some possible detections by other experiments.
Experiments attempting to detect dark matter focus mainly on one possible class, motivated by particle physics: WIMPs, or weakly interacting massive particles. The "weak" part of the name originally referred to the weak force, one of the four known fundamental forces in nature. However, WIMPs may or may not actually interact via the weak force—we know for certain dark matter interacts gravitationally, but whether it interacts with ordinary matter via any other force is currently unknown.
However, the gamble is that dark matter does interact in a limited way with normal matter, and detectors have been designed with that in mind. LUX consists of 368 kilograms of liquid xenon cooled to -110°C and surrounded by a tank of water. The whole apparatus is housed nearly 1500 meters (4,850 ft) underground in the old Homestake gold mine near Lead, South Dakota. The depth of the mine provides a lot of shielding against cosmic rays—high energy particles from supernovae and other astronomical sources—that could otherwise swamp the detector. WIMPs, on the other hand, should pass through the rock and reach the detector without much in the way of interaction.
Liquid xenon is much denser than water, so it provides an additional measure of shielding. Any particle piercing through to the center of the tank has likely avoided interacting with anything until that point. When such a particle collides with the nucleus of a xenon atom, it produces a recoil, emitting photons and electrons. Detectors lining the outside of the tank amplify those signals. Comparisons between the photon and electron timing provide a three-dimensional reconstruction of where the collision occurred and the energy involved. Together, these data reveal the mass and electric charge of the particle, telling researchers whether it could be dark matter or (more likely) something ordinary.


Even a very massive elementary particle has a small mass in terms of the measures we use in daily life: grams or kilograms (or slugs, if you want to get all American about it). Therefore, particle physicists frequently use the famous equation E = mc2 and write masses in units of energy instead. One electron volt (eV) is 1.783×10−36 kg; a proton's mass is about 0.94 GeV and the Higgs boson's mass is about 125 GeV.

LUX is designed to be sensitive to WIMPs across a wide range of possible mass values, overlapping the ranges other detectors are searching. Many theoretical models, including those motivated by supersymmetry (SUSY), predict "high-mass" WIMPs: those with masses greater than 35 billion electron volts (35 giga electron volts, or 35 GeV). However, the Cryogenic Dark Matter Search (CDMS) experiment in Minnesota found small but tantalizing hints of particles with masses around 8.6 GeV, which would fall into the "low-mass" WIMP regime.
This is why the new LUX results are interesting, despite showing nothing. For high-mass WIMPs, LUX has roughly twice the sensitivity of other detectors, but it also can detect particles in the low-mass regime. If the 8.6 GeV WIMPs seen in CDMS exist, three months of LUX operation should have found thousands of them. Yet LUX saw nothing. While it's still possible the CDMS particles could be real, they must interact in a way specific to the solid silicon-based detectors at CDMS and avoid collisions with xenon atoms—an unlikely possibility.
Given the increased sensitivity at LUX, the most popular prediction for a WIMP—the so-called lightest supersymmetric partner (LSSP )—is not looking good either. Depending on the particular model, the LSSP should have a mass greater than about 10 GeV, and LUX should have detected many of them. While the absence of particles is consistent with prior dark matter experiments, LUX's sensitivity makes it easier to say that high-mass WIMPs probably don't exist.
Of course, this result isn't a definitive statement on the existence of dark matter, but it does put stronger constraints on what its identity could be. Dark matter could still lurk at lower mass ranges, or it could be non-WIMP in nature. The latter possibility includes the disturbing prospect that dark matter doesn't interact directly with normal matter at all, except through gravity, dooming every current detection scheme to failure.
LUX will continue operating until 2015, and its planned successor—LUX-ZEPLIN, or LZ—should increase sensitivity to the point where even very rare dark matter collisions can be spotted.]

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