A wealth of observations shows that the universe is composed of >96% invisible matter and energy. The leading candidate for the invisible "dark matter" is a subatomic particle left over from the big bang known as the Weakly Interacting Massive Particle (WIMP). If WIMPs exist, they are also the dominant mass in our own Milky Way. Although they very rarely interact with conventional matter, they should nonetheless be detectable by sufficiently sensitive detectors on Earth, through their direct interaction with, and the ensuing recoil of, nuclei in a target. The primary detection challenge is reducing natural and cosmogenic radioactivity by >10 orders of magnitude.
This award will provide base support for the Case group for work on the LUX (Large Underground Xenon) dark matter experiment. LUX features a 300 kg two-phase xenon dark matter detector, which will be housed in a large water shield located in the Davis Cavern at the Sanford Lab in the Homestake Mine, South Dakota. LUX is currently beginning their final integration of the detector. The LUX experiment has focused on a scalable design based on existing technologies, and will soon field a detector with nearly 2 orders of magnitude better WIMP sensitivity than the best published limits. This group has had a central role in the design and construction of LUX, and will play a major role in LUX operations and analysis.
Among Broader Impacts, the detection of particle dark matter would transform and extend world activities in the particle physics and astronomical communities. The technology can be applied to other fundamental experiments such as double beta decay and solar neutrinos, and may give rise to new medical diagnostic techniques, or applications to Homeland Security and nuclear control. This group will participate in extensive education and outreach activities being undertaken in conjunction with Sanford Lab.
LUX is an experiment seeking to directly detect dark matter. Dark matter is the name given to some unknown form of matter which is thought to exist throughout the universe, and which has so far only been seen by its gravitational pull, but which is neither shining nor absorbing light. The most vivid demonstration of dark matter is in spiral galaxies, which have consistently been found to be rotating at speeds which should have long ago pulled them apart if the only gravitational "glue" holding them together comes from stars, planets, dust and gas in them. Like all galaxies, the Milky Way is thought to have roughly 10 times more dark matter than ordinary matter. But we don't know what the dark matter is. Astrophysical measurements tells us that it is not something made from ordinary atoms - either as gas or dust, or stars or planets, or even black holes. So it is natural to assume that it might be a new type of subatomic particle, but one that is not easy to see directly. We seek clues from the standard model of particle physics, and possible extensions to that model, such as supersymmetry. The most popular idea is that it is weakly interacting massive particles, or WIMPs. WIMPs would be extremely small - about the same size as neutrinos, or about a trillion times smaller than a nucleus, but have a mass about the same as a heavy atom. WIMPs would be left over from the big bang, and pervade our Milky Way with a density of roughly one per liter. They are orbiting in the galaxy and will occasionally scatter off of an ordinary atom here on Earth, depositing about as much energy as an X-ray. Thus, they can be measured in particle detectors of the sort used in particle physics and nuclear physics. The challenge is that these tiny particles mostly pass right through ordinary matter, interacting very infrequently. In most situations radioactivity will strike a detector 1 trillion times more often than the most optimistic expected rate from WIMPs. Over the last 25 years a series of experiments have been built to look for WIMPs. They have dealt with radioactivity primarily by using large lead shields to stop gamma rays from radioactivity in essentially any standard surrounding material such as concrete or rock. The detectors themselves are made of materials carefully selected and screened to have ultra-low levels of intrinsic radioactivity. As of yet, while there have been some controversial possible signals, the general consensus is that there is no firm evidence that WIMPs have been seen. Possibly, dark matter is not made of WIMPs, or perhaps we simply need larger detectors, because larger mass means more chance for a WIMP to interact, and lower backgrounds. The LUX experiment is fielding a new, larger mass and lower background experiment than has yet been done, based on a relatively new type of detector that features a vat of liquified xenon as the detection material. When a WIMP strikes one xenon atom in the detector, the resulting disruption causes a few hundred electrons to be stripped from xenon atoms, and a few hundred photons of ultraviolet light are emitted by excited xenon atoms. The photons are measured by a set of photon sensors known as photomultiplier tubes. The electrons are drifted to the top surface of the liquid xenon and extracted from it using electric fields, where they generate a second flash of ultraviolet light in xenon gas above the liquid. Measurement of the two flashes of light tells us the amount of energy in the original collision, and the ratio of size of the two flashes is different for WIMPs and most radioactive backgrounds. This is because WIMPs strike atoms whereas most radioactivity strikes electrons, and the two types of interactions generate different amounts of photons and free electrons. LUX is also the first dark matter experiment to use a large water shield instead of lead shield, which allows lower levels of backgrounds. It is being located 4850 feet underground in the recently decommissioned Homestake gold mine in the Black Hills of South Dakota. LUX will housed in the same cavern where solar neutrinos were first measured in a famous 30-year long experiment conducted by Ray Davis, who won the Nobel prize for this work in 2002. This former mine is the location of the newly created Sanford Underground Research Facility - SURF. During the funding from this award, the LUX collaboration completed construction of the LUX detector and successfully commissioned by operating it in a test run at facility located on the surface at SURF. At the end of this grant the detector was deployed in the final underground location, ready for the dark matter search in 2013.
