DarkSide: Project aims to find particles of dark matter

The DarkSide-50 research team is made up of faculty, students and researchers from dozens of institutions around the world,

From left, Luca Grandi, an assistant professor at the University of Chicago, Richard Saldanha, an associate fellow in the Kavli Institute for Cosmological Physics at the University of Chicago, and Hanguo Wang, a researcher at the University of California-Los Angeles. 

While wearing protective clothing to keep the environment clean, they are working to assemble the core of the dark matter detector, an argon-filled tank with photodetectors at the top and bottom to spot the light from collisions, and copper coils to help determine where the collisions occur. 

Credit: Peter Meyers

In a laboratory under a mountain 80 miles east of Rome this fall, a Princeton-led international team switched on a new experiment aimed at finding a mysterious substance that makes up a quarter of the universe but has never been seen.

The experiment, known as DarkSide-50, is searching for particles of dark matter. For the last several decades, researchers have known that visible matter—the stuff we can see—makes up only 4 percent of the universe, while dark energy is thought to make up about 73 percent.

Dark matter is thought to make up the remaining 23 percent, and finding it, researchers say, will solidify our understanding of how the universe formed and shed light on its ultimate fate.

Peter Meyers

"This is like the search for the Higgs boson was 10 years ago," said Peter Meyers, a professor of physics at Princeton University and one of the lead scientists on the project.

"We have a good idea of what to look for, but we don't know exactly where or when we will find it."

Housed inside a cavernous chamber in Italy's Gran Sasso National Laboratory, the DarkSide-50 collaboration involves 17 American institutions as well the Italian Institute for Nuclear Physics (INFN) and other institutions in Italy, France, Poland, Ukraine, Russia and China.

The research team includes postdocs, staff researchers and several graduate and undergraduate students from Princeton.

The researchers spent last summer assembling the detector, which consists of three fluid-filled chambers nested one inside the other like Russian dolls.

Now that the experiment is up and running, the waiting begins. Unlike the massive Large Hadron Collider that discovered the Higgs, DarkSide-50 doesn't smash anything. Instead, it is designed to detect dark matter particles that drift through its chambers.

Looking for WIMPs 
The evidence for dark matter dates to the 1930s, when astronomers realized that the amount of matter we can see—as planets, stars and galaxies—falls far short of what must be out there to give galaxies their characteristic spiral shapes and clustering patterns.

Without this missing matter, the galaxies should have flown apart long ago. Matter provides the gravity that keeps the stars in rotation around the galaxy's center.

Unless our theories of gravity are wrong—and a minority of physicists think that is a possibility—dark matter must exist.

Cristiano Galbiati

"Finding dark matter particles would help confirm our understanding of the universe," said Cristiano Galbiati, an associate professor of physics at Princeton.

"And, whether or not we find it, we will have learned a great deal about how to go about looking for it. This is as exciting a moment in the search for dark matter as there has ever been."

Although no one knows for sure what dark matter is made of, the DarkSide-50 team and many other scientists think the most likely candidate is a particle so weak that it is called a WIMP, which is short for "weakly interacting massive particle."

As the name suggests, WIMPs barely interact with their surroundings. They simply drift through walls like ghosts.

If you cup your hands together, you will surround—but never trap—a few of these ethereal beings. Scientists suggest that a WIMP can be detected when it smacks into the nucleus of an atom such as argon, which is found in air.

When this happens in a chamber of densely packed argon atoms, the stricken atom recoils and creates a track of excited argon atoms in its wake.

This track appears as a fleeting trail of light, which can be detected by devices called photodetectors. But these collisions are rare—just a few WIMPs are detected each year.

Because other particles also give off light when they collide with argon, DarkSide-50 is located nearly a mile beneath Gran Sasso mountain ("gran sasso" is Italian for "great stone").

The rock shields out cosmic-ray particles that routinely bombard the Earth.

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