The Large Underground Xenon (LUX) experiment is trying to identify the nature of dark matter, an invisible substance that physicists believe is all around us, making up most of the matter in the universe, even though it has effect on our lives.
The umbrella term 'dark matter' encompasses about 25% of the Universe, while what we know as matter makes up about 5%. The rest consists of what is called "dark energy" and no one knows anything about that other than that it is something helps make gravity behave strangely at the very large scale.
The first results are to validate the LUX experiment's design and performance - and that is a challenge to papers that have claimed 'sightings' of dark matter. Seventeen universities and research institutes in the USA, Portugal and the UK run the experiment, situated in a former gold mine 1.5 kilometers below the Black Hills of South Dakota. Work on LUX started in 2008 and the experiment was ready for an initial run earlier this year. Most of the funding is from the National Science Foundation and the US Department of Energy. The good news is that it worked as expected when they turned it on.
They're working in such an extremely sheltered environment because they are looking for tiny and extremely rare flashes of light that would indicate a collision between a dark matter particle and a normal matter particle. Since the experiment was installed underground in February, they have been looking for Weakly Interacting Massive Particles (WIMPs), which are the prime candidates to constitute the dark matter in our galaxy and in the Universe. These particles are thought to have mass like normal particles and create a tiny gravitational pull, but cannot be observed directly since they neither emit nor absorb light at any wavelength. On the largest scales, its presence can be inferred from the motion of stars within galaxies, and of individual galaxies in galactic clusters.
Collisions between WIMPs and normal matter are rare and extremely difficult to detect because cosmic-ray particles from space can overwhelm the already faint flashes expected from WIMPs. Few cosmic rays can penetrate as deep underground as the LUX experiment and the detector is further protected from background radiation by being immersed in a shielding tank of ultra-pure water.
"To give some idea of how small the probability of having a dark matter particle interact, imagine firing one dark matter particle into a block of lead," said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for LUX. "In order to get a 50-50 chance of the particle interacting with the lead, the block would need to stretch for about 200 light years — this is 50 times farther than the nearest star to the Earth aside from the sun. So it's an incredibly rare interaction."
Theory and experimental results suggest that WIMPs could take either a high-mass or low-mass form. In the search for high-mass WIMPs weighing 40 times the mass of a proton, LUX has twice the sensitivity of any other dark matter direct-detection experiment, according to these new results. LUX also has greatly enhanced sensitivity to low-mass WIMPs, and new results suggest that potential detections of low-mass WIMPS by other dark matter experiments were likely the result of background radiation, not dark matter.
"There have been a number of dark matter experiments over the last few years that have strongly supported the idea that they're seeing events in the lowest energy bins of their detectors that could be consistent with the discovery of dark matter," said Gaitskell. "With the LUX, we have worked very hard to calibrate the performance of the detector in these lowest energy bins, and we're not seeing any evidence of dark matter particles there."
"LUX has significantly higher sensitivity than the previous world's best dark matter experiments – especially for the lightest WIMPs, which cause the faintest signals," says Dr. Henrique Araújo from the Department of Physics at Imperial College London.
The new LUX result challenges evidence from other experiments, such as CoGeNT and DAMA, where scientists have previously claimed to have data about the nature of WIMPs.
Araújo says, "A number of previous results make it look like WIMPs exist with a particularly low mass. While this may still turn out to be the case, our new data reveal that, on that occasion, it was a case of mistaken identity."
HOW DOES IT WORK?
At the heart of the experiment is a 6-foot-tall titanium 'thermal flask' filled with almost a third of a ton of liquid xenon, cooled to minus 100 degrees centigrade.
When a WIMP hits a xenon atom it recoils – like a white billiard ball striking the opening triangle of colored balls in a game of snooker – and photons of light are emitted; at the same time, this interaction also releases electrons from surrounding atoms.
The electrons are drawn upward by an electrical field and get absorbed into a thin layer of xenon gas at the top of the tank, releasing more photons.
Light detectors in the top and bottom of the tank are each capable of detecting these two photon signatures. The locations of the two signals can be pinpointed to within a few millimeters.
The energy of the interaction can be precisely measured from the brightness of the pulses of light. Any particles interacting in the xenon will cause these signals, but WIMP interactions are expected to have characteristic sizes which are quite different from those caused by ordinary particles.
Scientists are already designing, and soon will start building, the next-generation experiment, LZ, which is the coming together of two programs – LUX and its predecessor ZEPLIN, the first dark matter detector of this type underground at the Boulby mine in North Yorkshire.
With a 7-ton liquid xenon target, LZ will be 30 times larger than LUX and have over 100 times better range. It will be so sensitive that it will be limited only by the interference of background signals from astrophysical neutrinos. These similarly illusive particles were once a candidate to explain the dark matter problem — but physicists now know they are not massive enough to do the job.
With LUX's initial run complete, the team will now make a few adjustments to fine-tune the device's sensitivity in anticipation of a new 300-day run to begin in 2014, where they hope to definitively rule out a vast swath of parameter space where dark matter might be found.