Researchers at the Sanford Underground Research Facility in Lead, South Dakota, go deep underground to try to answer some of the most challenging physics questions about the universe. What is the origin of matter? What is dark matter and how do we know it exists? What are the properties of neutrinos?
Sanford Lab is located at the former Homestake gold mine, a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis first recognized the potential for deep science in the mid-1960s when he built his solar neutrino experiment at Homestake. In 2002, his ground-breaking research earned him the Nobel Prize in Physics.
Why do scientists go so deep underground to study particles that come from the universe? Hold out your hand. On the surface, millions of cosmic rays pass through it every few months. But nearly a mile underground, it’s a million times quieter! The rock acts as a natural shield, blocking most of the radiation that can interfere with sensitive physics experiments. It turns out Sanford Lab is particularly suited to large physics experiments for another reason—the hard rock is perfect for excavating the large caverns needed for big experiments.
Barrick Gold Corporation, which owned Homestake mine, closed the facility in 2001. Five years later, the company donated the property to South Dakota for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. Since then, the State has committed more than $45 million in state funds to the project. Early on, South Dakota received a $10 million Community Development Block Grant to help rehabilitate the aging facility.
In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.
In December 2010, when the National Science Board decided not to fund further design of DUSEL, physicists and citizens alike began seeking other sources. In 2011, the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments.
The first two major physics experiments at Sanford Lab are 4,850 feet underground in the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) dark matter experiment is housed in the same cavern excavated for Ray Davis’ experiment in the 1960s. Dark matter makes up most of the matter in the universe, but interacts so rarely that researchers call the particles WIMPs (weakly interacting massive particles). So, how do we know dark matter exists? Because of its gravitational effects on galaxies and clusters of galaxies. Scientists with the Large Underground Xenon (LUX) experiment use a cryostat filled with 1/3 of a ton of liquid xenon to look for these elusive particles. They believe that when a WIMP strikes a xenon atom, the electrons will release more photons, leaving a very particular signature. In October 2013, after an initial run of 80 days, LUX was named the most sensitive dark matter in the world. The experiment completed a 300-live-day run and is now analyzing the data collected. In 2016, LUX will be decommissioned to make way for LUX-Zeplin (LZ). This second-generation sark matter experiment will hold 7 tons of liquid xenon and be up to 100 times quieter than LUX. It will be located in the same cavern.
Neutrinos, among the most abundant particles in the universe, are often called “ghost” particles because they pass through matter like it isn’t there. And as they travel, they change “flavors,” from electron to muon, from tau to electron, from muon to tau and so on. They also have very little mass. Scientists with the MAJORANA experiment want to know more about the properties of neutrinos. They also hope to find a rare phenomenon called neutrinoless double-beta decay, which could tell them if neutrinos are their own anti-particles. The answer to this question could help us understand why humans—and, indeed, the universe—exist. Built almost entirely out of copper electroformed deep underground at Sanford Lab, MAJORANA uses dozens of detectors made of enriched germanium crystals (76Ge) in its quest.
Another major experiment, the Long Baseline Neutrino Facility and associated Deep Underground Neutrino Experiment (LBNF/DUNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab—is in the preliminary design stages Researchers with DUNE also want to learn more about neutrino behavior. To get precise measurements of neutrino oscillation they are building the Long Baseline Neutrino Facility (LBNF). Called the “next frontier of particle physics,” LBNF/DUNE will follow neutrinos as they travel 800 miles through the earth, from Fermilab in Batavia, Ill., to Sanford Lab. The detector will also help them study the neutrinos that are created when a star goes supernova.
Sanford Lab also hosts experiments in other disciplines—including biology, geology and engineering, or, as we like to call them, the bee-gees. Experiments include the search for life underground, gravitational waves and seismic activity.
Additionally, the Black Hills State University Underground Campus offers students from universities across the country opportunities to develop research projects in a variety of disciplines, including physics. Currently, the campus houses a Low-Background Counting Facility that documents the levels of radioactivity in materials that will be used in experiments around the world.
About the header picture: Using the world’s purest copper, members of the Majorana Demonstrator collaboration built their experiment inside a glove box on the 4850 Level of Sanford Lab. Photo by Matt Kapust, Sanford Underground Research Facility.