Tag Archives: LUX

Another twist in the dark matter mystery

In New Eyes on the Universe I gave only passing mention to the Large Underground Xenon (LUX) dark matter experiment. LUX was clearly going to be an important player in the search for dark matter, but while I was writing the book the experiment was still in its commissioning phase. Yesterday, LUX presented results from its first three months of operation. (For those who haven’t read the book, LUX employs 370kg of liquid xenon cooled to about 160K and shielded by water in a search for WIMP dark matter. If a WIMP collides with a xenon nucleus then the photons and electrons emitted as the nucleus recoils can be detected. In order to shield the xenon from cosmic rays and other background radiation, experimenters have placed the detector a mile underneath the Black Hills of South Dakota.)

The first thing to note is that LUX is now the world’s most sensitive detector currently searching for WIMP dark matter. Richard Gaitskell, a spokesperson for LUX, described its sensitivity with a footballing analogy. Imagine a 75000-strong crowd of football fans, each clapping twice a second: the number of claps is what  the detector was hearing each second while it was on the surface. That’s a tremendous cacophony. When the detector was placed a mile underground it was  as if the clapping fell to a rate of one clap per minute. That reduction in background is necessary: LUX is trying to ‘hear’ the equivalent of a sigh…

The second thing to note is that LUX is sensitive to WIMPs across a wide range of possible masses. There have been tantalizing hints by other dark matter experiments of WIMPs having a relatively low mass of around 8.6 GeV; many models based on supersymmetry, on the other hand, predict WIMPs with a mass of 35 GeV or more. LUX is sensitive to both low- and high-mass WIMPs.

And the results of the first 90 days of LUX operation? Well, the LUX data are consistent with the detector having seen zero dark matter particles during that time. As a LUX team member put it: “We’ve seen nothing better than anyone else.” The problem is, if an 8.6 GeV WIMP particle did indeed exist, as hinted at by CDMS, then LUX should have seen 1550 of them during those first 90 days. It seems impossible to reconcile these latest results with the existence of a low-mass WIMP.

The LUX results don’t prove the non-existence of dark matter, of course, and before reaching any conclusions we will really need to wait for the next LUX report: that will present an analysis of the first 300-days of operation. But the LUX results do put the dark matter mystery squarely in the spotlight: it’s becoming imperative that we learn just what dark matter is.

Fiat LUX

One of the several dark matter detectors I describe in New Eyes on the Universe is the Large Underground Xenon (LUX) detector. The search for dark matter is much more difficult than the search for the Higgs, but the detectors such as LUX should eventually find WIMP dark matter particles – if such particles exist.

Well, today LUX took a step closer to its goal. For the past two years, scientists have been testing the 3-tonne detector (which contains 350 kg of liquid xenon) in a laboratory. Today, they transferred it to its permanent home 1500m below ground in the old Homestake gold mine – site of the famous Davis solar neutrino experiment. It was a delicate operation: the detector was taken down on air bearings in order to protect it from even minor bumps. But the operation was a success. The detector is in place, and it will start taking data later this year.

Schematic of the LUX detector

A schematic of the LUX detector (Credit: Symmetry magazine)

The mile or so of rock above the detector will shield it from cosmic rays, but of course dark matter particles will pass through the rock as if it weren’t there. The hope is that once in a while a dark matter particle will collide directly with a xenon nucleus in LUX. When xenon is hit by a particle (it could be a photon, a neutron, or a dark matter particle), liquid xenon both scintillates and ionizes. By using sophisticated detectors that surround the xenon, physicists can measure the ratio of scintillation over ionization energy from the collision. And from that information they can determine the type of particle involved in the collision – whether photon, neutron or dark matter. That’s the hope, anyway. By this time next year we should know more.