Dark matter, written into theory to explain the behavior of massive celestial objects far above us, could be detected by heading down below—nearly a mile into the Earth. That’s the hope, anyway, of an experiment scheduled to begin next year in a South Dakota gold mine that closed in 2002.
The Sanford Underground Laboratory, dedicated this week 4,850 feet (1.5 kilometers) belowground at the Homestake Mine in Lead, S.D., will be home to the Large Underground Xenon (LUX) dark matter detector, among other experiments. Dark matter is the mysterious, invisible stuff believed to contribute roughly six times as much mass to the universe as does ordinary matter—the atoms, molecules and structures of everyday life. Its effects have been seen in its gravitational pull on large-scale structures in the universe, but its true nature remains unknown.
LUX will benefit from the Earth’s shielding from cosmic radiation as it looks for the minute collisions hypothesized to occur between ordinary atoms and dark matter particles. A prevailing theory of dark matter holds that the particles are WIMPs—weakly interacting massive particles—which interact only via gravity and the weak nuclear force, the force responsible for particle decay. “We’re looking for the very occasional interaction of dark matter with ordinary material,” says Brown University physicist Richard Gaitskell, co-spokesperson for LUX.
That ordinary material is about a third of a ton (350 kilograms) of liquid xenon, shielded by a massive water tank and watched by photomultiplier tubes and charge detectors for any photons or electrons produced as dark matter passes through the detector and scatters off a xenon nucleus.
Gaitskell says the LUX team plans to move the experiment onto the Sanford site this year and to start taking data by the middle of next year.
Whatever the outcome, Homestake already has a particle-physics pedigree: the mine was home to the late physicist Raymond Davis’s experiments that began in the 1960s. Davis used an even larger liquid tank than that of LUX to detect other lightly interacting particles, neutrinos produced in the sun, in work that earned him a share of the 2002 Nobel Prize in Physics.
Photo of South Dakota governor Mike Rounds [second from left] and namesake donor T. Denny Sanford [center] at this week's dedication: Bill Harlan, Sanford Laboratory at Homestake