UA Researchers to Embark on New Search for Mysterious Dark Matter
TUSCALOOSA, Ala. — Three University of Alabama physicists are among dozens of researchers planning and developing a new international search for dark matter – invisible material scientists believe makes up roughly 27 percent of the universe’s mass.
The project, on which the UA scientists have collaborated since near its 2012 outset, received a boost last month when the Department of Energy and the National Science Foundation announced it was one of three new dark matter projects selected for support.
Dark matter is known to exist, scientists say, partly because they see the influence of invisible materials in gravity, including both within and around our own Milky Way galaxy.
“It’s like a soup in which we’re living,” said Dr. Andreas Piepke of dark matter.
Piepke, a UA professor of physics and astronomy, along with physics colleagues Drs. Jerry Busenitz and Ion Stancu, are University representatives on the international project managed by the Lawrence Berkeley National Lab.
The soup analogy is, perhaps, most apparent when thinking of the Earth’s location in space, positioned within one of our galaxy’s rotating spiral arms and influenced by dark matter both within, and well outside, our atmosphere.
One of the problems associated with an aspect of the Big Bang Theory – the idea that the universe erupted from a central location and its galaxies have been moving apart from one another since – is scientists would expect this movement to slow over time, Piepke said. Instead, it’s accelerating. The presence of dark energy offers an explanation.
In the newly endorsed Lux-Zepelin, or LZ, experiment, scientists will search for one potential type of dark matter particle – something called WIMPS, or weakly interacting massive particles.
“We will use seven tons of liquefied xenon to look for the existence of this unknown type of matter,” Piepke said.
All that xenon will be housed in a titanium tank positioned nearly one mile underneath the surface within the Sanford Underground Research Facility in South Dakota, the site of a former gold mine. Surrounding the tank will be the high-tech instrumentation of the particle detector.
The underground location is necessary to help shield the detector from cosmic rays that repeatedly shower the Earth and would serve as unwanted “noise,” to the scientists looking for collisions between a xenon atom’s nucleus and WIMPs.
“The detector is built to look for that collision,” Piepke said. When such a collision occurs, the two items bump together like billiard balls then move apart, resulting in a transfer of kinetic energy from the dark matter particle to the xenon atom.
“That energy deposit is what we’re looking for,” Piepke said. “That is the signature for the detection of dark matter.”
Those collisions are infrequent.
“Dark matter’s existence might be manifested by something that occurs only a few times over an interval of several years,” Busenitz said. This is why, he said, the tank contains such a large volume of xenon – increasing the number of xenon atoms increases the probability of a collision.
“It’s like if you were walking down the street, and if there’s one person per block, the probability that you’re going to run into them is smaller than the probability if there were 100 people per block,” Busenitz said.
The experiment is tentatively scheduled to begin in 2018. First, scientists and engineers must build the detector, and the UA physicists are already actively filling a critical role in that process.
All material used in the detector’s construction must be tested to ensure it does not exceed the acceptable thresholds of radioactivity and radon as too much would interfere with the project’s particle detection efforts. Highly sensitive, custom-built germanium detectors at The University of Alabama are already testing candidate materials.
One such UA device, Piepke said, can count concentrations of uranium and thorium – naturally occurring radioactive elements – on a level as small as .1 parts per billion.
The experiment’s selected data centers will be tasked with recording, processing and storing huge amounts of data from the experiment. Stancu is also leading a task force that will establish the criteria the candidate data centers must meet for selection.
The UA researchers will also custom-make sources which emit gamma rays and neutrons to help calibrate the detector prior to the experiment’s data collection phase. This, too, helps the scientists differentiate between atomic collisions in which they are and are not interested.
The research group is composed of 128 researchers from 29 institutions in the U.S., the United Kingdom, Portugal and Russia.
UA’s department of physics and astronomy is part of the College of Arts and Sciences, the University’s largest division and the largest liberal arts college in the state. Students from the College have won numerous national awards including Rhodes Scholarships, Goldwater Scholarships and memberships on the USA Today Academic All American Team.
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