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U. physicists install underground detector in search of clues about dark matter

An international collaboration led by physicists from the University installed an underground detector in September with the hope of discovering a new fundamental particle that could account for dark matter. The detector is currently located in Gran Sasso National Laboratory in Italy, where it will remain for the duration of the experiment.

This detector is part of the DarkSide-50 experiment that is looking for weakly interacting massive particles (WIMPs), particles that could account for the large amount of mass that seems to be missing from the universe. WIMPs are postulated particles, meaning that they may not actually exist. Physics professor Peter Meyers, one of the lead scientists on the University’s team, explained that while astrophysicists can see the gravitational effects of dark matter, dark matter does not emit light or interact with ordinary matter.

“All the studies we’ve ever done, everything we’ve ever thought of, has been on some minor component of the universe, which clearly is kind of earthshaking from a philosophical point of view. So it immediately becomes one of the biggest questions in science,” Meyers said.

Physics professors Frank Calaprice and Cristiano Galbiati led the team with Meyers. Galbiati noted that dark matter is the “one area of physics that is not understood,” even though physicists have observed its effects for the past 70 years. He said that it is not surprising that the most basic questions about dark matter have not been explained, since periods of uncertainty are “part of the normal progress of science.”

Calaprice could not be reached for comment.

Meyers noted that while this experiment aims to enhance the fundamental understanding of our universe, the detector itself is only about a foot high.

“The fact there was cutting edge science being done with an apparatus that small … was very appealing,” he said.

In 2007, the particle astrophysics group at the University completed a small prototype of the detector that was able to produce close to the best sensitivity in the world for WIMP detection. Although the device itself remains very small, it is housed in a laboratory the size of a three-story building and the project involves an international collaboration of engineers, technicians and specialists, Meyers said.

Now that the detector has been built and set up for the experiment to begin, Galbiati explained, the original group is collaborating with specialists in Italy and other universities across the United States, although Princeton remains at the heart of the effort.

Meyers, who did not join the particle astrophysics group until 2007, explained that he has led the effort to design and build the time projection chamber, the core of the detector which measures the positions of WIMPs.  The chamber is filled with liquid argon, which produces a signal when a WIMP collides with the argon nuclei.

Meyers explained that one of the many benefits of this small, University-driven experiment was its accessibility to both undergraduate and graduate students.

“You can step into a position of great responsibility very quickly,” he said.

Jason Brodsky, a fifth-year graduate student of physics, has been working with the DarkSide team since its very early stages. He spoke of his work with a portion of the experiment that translates signals from liquid argon into visible light that can be picked up by the light detectors.

“I was the person who knew how to operate that chemical coding machine, and so I went to Italy and spent a lot of time on my feet in the clean room operating that machine,” Brodsky said.

Brodsky explained that his work ranged from very basic mechanical tests to activities that involved expert knowledge of the physics behind the experiment.

“Physics doesn’t happen on a different plane from the rest of the world,” he said. “I think that there are a lot of aspects of our job that are really approachable. It’s a little bit of a shame we have so few opportunities to show the inside of the experiment to people outside of physics.”

“I joke a lot of my job is plumbing,” he added.

Will Taylor ’14, a senior in the physics department, travelled with the DarkSide team to Italy to engage in research for his senior thesis.  He worked with Calaprice on developing and testing an apparatus to remove background radioactivity from the layer of water surrounding the detector. Taylor explained that due to the great sensitivity required to detect a WIMP, the layer of water shields the device from other cosmic rays that could interfere with detection of the desired signal.

Brodsky explained that the experiment is currently in a “calibration phase,” adding that the physicists analyzing the data from the DarkSide-50 need to be sure that they are able to separate a true WIMP signal from other signals, such as those caused by trace radioactivity.  Meyers explained that the team is currently using high-radioactivity atmospheric argon to ensure that these background signals can be distinguished. After several months, the detector will switch to low-radioactivity argon sourced from underground wells, and the physicists will begin to look for signals from these new fundamental particles, he said.

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