The hunt for one of the universe’s greatest unknown particles
The Soudan Underground Laboratory rests deep in the earth—more than 2,000 feet below the surface—in an inactive mine in northern Minnesota. Today, it is one of the leading underground science and engineering laboratories in the United States, and Martin E. Huber, PhD, professor of physics and electrical engineering at the University of Colorado Denver, has worked among a consortium of researchers from several institutions over the last 10 years, in the search for the elusive "dark matter"—most easily defined as one or more undiscovered fundamental particles of physics beyond known theory, but something that physicists hypothesize makes up the majority of our universe.
Late last week, Science magazine published findings from Huber and colleagues involved in the Cryogenic Dark Matter Search (CDMS) experiment, which indicate they have detected two events deep down in the earth within the 125-year-old mine that are consistent with what scientists would expect to occur when an interaction with one specific dark matter candidate—particles collectively known as weakly interacting massive particles or "WIMPs"—transpires.
Virtually undetectable because it neither reflects nor absorbs light and only rarely collides with "normal" matter (what we know as solid objects and known gases), dark matter is present all throughout the universe. Originally hypothesized to exist by an astrophysicist in 1933, scientists have spent the last 80 years or so in search of an understanding of exactly what dark matter is—what is this stuff that makes up the rest of a galaxy besides the stars, planets and gases? What are the unseen particles that keep a galaxy from flinging apart in its very movement? What is it that constitutes more than 80 percent of the mass of the universe?
Physicists use proven theories of gravity along with computations of masses of the stars in a particular galaxy to determine expected stellar trajectories and overall mass, and it just doesn’t add up—something else has to be out there holding it all together beyond gravity and we can’t see it. Not only that, but even though we can’t see it or feel it (it doesn’t bounce off anything like other known forms of matter would), dark matter is expected to be all around us, all of the time, floating through us and everything around us. Through many indirect astrophysical observations, there is little doubt—and compelling evidence—that it exists.
The experiment in the Soudan mine uses some of the most advanced methods to-date to try to detect and determine the properties of dark matter. Hidden deep inside the earth, which acts as a filter for any cosmic rays (or normal particles from space) that might mimic a WIMP signal, a collection of detectors approximately the size of hockey pucks and made from germanium crystals sits silently and patiently waiting for a rare occurrence when a WIMP might collide in just the right way with a germanium atom to deposit some of its energy in the crystal. Dedicated computers monitor the detectors every second of every day, recording all disturbances, regardless of their source.
Theoretically, WIMPs are passing through the detectors (even the entire planet) all of the time, and germanium presents one of the best opportunities scientists have to observe the rare interaction of a WIMP with normal matter. Such an interaction would create a miniscule vibration, or a very tiny amount of heat; causing an associated rise in temperature that signals the collision of a WIMP and atom inside the crystal detectors.
The CDMS researchers cool the crystals to nearly absolute zero (a chilling -460 degrees Fahrenheit) so that there is virtually no energy in the atoms. "The atoms of the germanium crystals are nearly motionless at this temperature," explains Huber. "And, sitting more than 2,300 feet below the Earth’s surface, where most external events are blocked, we have the opportunity and sensitivity to identify any WIMP interactions that might occur."
Huber likens a potential WIMP interaction to the very pure, quiet ring that occurs when you tap a crystal glass. In this case, the WIMP "taps" the germanium nucleus, causing the nucleus to recoil or "ring". Just as the sound can be converted to an electronic signal by a microphone, the nucleus recoil is converted to an electronic signal by very sensitive thermometer attached to the germanium crystal. The properties of that signal allow the researchers to distinguish the difference between types of interactions—looking for that rare signal that has properties consistent with a WIMP collision. After reviewing the data, the researchers have observed two instances of this type of collision and have concluded they are possible candidate events for dark matter.
"While we can't say for sure that we ‘saw’ dark matter, we also can't say for sure that we ‘didn't’ see dark matter," added Huber. "All we know is that two events occurred that are consistent with a dark matter interaction; we'd need to see many more such events before we could claim a ‘discovery’. Either way, it’s a very exciting time for us and for everyone else who is watching and waiting for proof of the direct detection of dark matter."
Huber said the next step for the CDMS researchers is to build a bigger and more sensitive detector, one that has the potential to observe three, four, five—or more—candidate events that will bring them one step closer to being able to say "Yes, we have found dark matter." Right now, it’s a bit of a scientific race—with bragging rights and a likely Nobel Prize for the fortunate team of scientists who directly observe dark matter first in a terrestrial experiment. Huber is optimistic; in fact, one of the two candidate events occurred on his birthday. "Is this meaningful?" Huber asks. "No, of course not—but it sure makes a great story."