Professor of Physics and Astronomy, IU South Bend
A meteorite struck the Earth and created Canada's Sudbury basin some 1.8 billion years ago. It is there that scientists are listening for the sound of WIMPS, which researchers hope will one day confirm the existence of dark matter.
IU South Bend physicist Ilan Levine is not your typical sound engineer. His recording studio alone is of note, located 50 miles north of Lake Ontario in Canada inside the deepest underground lab facility in the world.
Then there is his orchestra, a contingent of weakly interacting massive particles (WIMPs), a mysterious, still-hypothetical group of players whose presence physicists like Levine hope will one day confirm the existence of dark matter.
If WIMPS are these particles that only interact via gravity and weak nuclear forces -- but not through electromagnetism or strong nuclear forces, thus remaining invisible to and passing undetected through atoms -- Levine thinks these WIMPs should be making some noise.
Working within two separate groups of scientists, one purely U.S.-based and the second consisting of members from the U.S., Canada and Czechoslovakia, Levine's task -- and fascination -- is to capture this sound, however short and subtle.
The soundscape, scientists believe, should reach a crescendo as WIMPs pass through super-heated liquids being held in specialized chambers housed at SNOLAB, which is more than one mile below the earth's surface in a 50,000-square-foot expanse inside an active nickel-copper mine. It is here, near Sudbury, Canada, that the experiment can be protected from more strongly interacting space particles like cosmic rays.
SNOLAB, which is also known as the Sudbury Neutrino Observatory, was first identified in 1984 as an ideal site to conduct early neutrino experiments because of the remote mine caverns created when companies extracted the rich seams of nickel and copper ore left within reach, scientists believe, when a meteorite struck the Earth and created the Sudbury basin some 1.8 billion years ago.
Using supersensitive ultrasonic transducers designed and built at IU South Bend, scientists hope to capture the sound a dark matter particle makes on the rare occasions when it hits the nucleus of an atom of the superheated liquid. The recoiling nucleus deposits a burst of heat which triggers an explosive transition to gas which emits an acoustic signal lasting only about four milliseconds.
The international experiment, called PICASSO (Project In CAnada Searching for Supersymmetric Objects), disperses droplets of superheated freon in a gel matrix designed to keep impurities and rough surfaces from providing protobubbles, rather than WIMPs. In November, the PICASSO project completed installation of 32 freon-containing detectors in the cavern. Now the tricky part is to sort out these supposed dark matter "pops" with competing sounds made by that ever-so-common leftover of radioactive decay, alpha particles.
Excitement has never been greater for scientists because a month earlier PICASSO made an important discovery when the detectors were able to identify what scientists determined were significant differences in the amplitudes of the acoustic sounds made between alpha particles and neutrons that were introduced to the superheated liquids. These neutron-induced signals are believed to be almost identical to dark matter signals.
Using sound technologies like controlled acoustic coupling, development of an improved transducer system and reducing sound gain to avoid saturation effects, scientists now believe they can use background suppression techniques to better discriminate between WIMP-like sounds and those from alpha particles.
"The results were so exciting because they were the first indication that we could distinguish the most pernicious background signal -- alpha particles from radioactive decay -- from WIMPs by details of their acoustic signals on an event-by-event basis," Levine said. "This was discovered quite accidentally during a normal process of detector calibration with our improved modules. We are now doing R&D on the acoustic transducers to improve our ability to separate this background from the signals we are searching for. The separation effect will most likely be even greater for the COUPP experiment."
COUPP (Chicagoland Observatory for Underground Particle Physics) is the solely U.S.-managed experiment in which Levine is involved. It has taken the route of designing radioactivity-free, ultra-smooth containers that can hold the superheated liquids within a single container.
The first detector held 1.5 kilograms of superheated CF3I, a fire-fighting chemical, in a one-liter container that led to results reported in the journal Science last year that closed the last door on a WIMP interpretation of the one experiment that claimed to have observed dark matter.
Now COUPP is preparing for data-gathering with the next generation of detectors, ones that total about 100 kilograms of CF3I, at underground sites in Chicago and the U.S. Department of Energy's Fermi National Accelerator Laboratory. Levine's students constructed various components for these detectors, most notably submergible acoustic transducers and preamplifiers which have been used to mitigate the background effects of high-energy neutrons associated with cosmic rays.
Levine has worked on these projects with William Feighery, an IU South Bend chemistry professor, and IU South Bend students Eric Abarbanell, Ryan Bauernfeind, Edward Behnke, S. Rey Brandt, Eric Greiner, Cynthia Muthusi, Earl Neeley, Tina Shepherd, Naomi Tankersley and Nathan Vander Werf. Cornell University student Henry Hinnefeld, Purdue student Joshua Behnke and Marian High School student Inyoung Park have also worked in Levine's laboratory.
Detecting WIMPs would be the greatest step taken to defining dark matter, a substance that could constitute at least some of the 85 percent of all matter in the universe that physicists believe does not consist of atoms. Only about 15 percent of the universe is known to consist of atom-based stuff like animals, planets and even black holes.
To date, Levine has developed more than 300 ultrasonic transducers for the PICASSO dark matter experiment, along with numerous acoustic transducers, preamps and cameras for the COUPP project.