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Detecting Dark Matter

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.

"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."


Photo: Princeton senior Maria Okounkova (left) and Kirsten Randle, a junior at the  University of Massachusetts-Amherst, prepare equipment for use in the  DarkSide-50 detector. They are wearing protective clothing to keep the  environment clean. (Photo by Yury Suvorov, University of California-Los  Angeles)

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.

"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."

Photo: Frank Calaprice, Photo by Theodore H. Lewis III, Department of Physics

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.

Photo : Cristiano Galbiati, Photo by Theodore H. Lewis III, Department of Physics

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.

"Separating the rare WIMP events from background is the main  challenge of all dark matter experiments," said Princeton physics  professor Frank Calaprice, who leads the project with Meyers and Galbiati.


"DarkSide is an attempt to build an apparatus that is as close as possible to an  ideal 'background-free detector.' The design benefits from methods and  two decades of experience that the Princeton group had developing the  Borexino solar neutrino experiment," he said, referring to another  experiment at Gran Sasso that ultimately achieved record-breaking low  backgrounds and the detection of rare solar neutrinos, which are  elementary particles that were theorized to exist long before they were  detected.

Photo: Peter Meyers, Photo by Theodore H. Lewis III, Department of Physics


The researchers began collecting data on collisions in November, but  it will take some time before they are ready to publish any results.  Demonstrating that they have achieved the desired level of sensitivity  will be an important finding even if WIMPs are not detected, said  Galbiati.

The investigators plan to increase the sensitivity this summer when  they swap out the current batch of argon, which was collected from the  air, with argon from a mine in Colorado. The underground argon contains  150-times fewer naturally occurring radioactive isotopes, so using it  will reduce the "background noise" even further.

Photo: Princeton senior Maria Okounkova (left) and Kirsten Randle, a junior at the  University of Massachusetts-Amherst, prepare equipment for use in the  DarkSide-50 detector. They are wearing protective clothing to keep the  environment clean. (Photo by Yury Suvorov, University of California-Los  Angeles)

DarkSide-50 is located in one of three caverns at Gran Sasso. The  WIMP detector itself is about the size of a grocery bag, and contains 10 gallons of pure argon that has been liquefied by cooling it to minus  186 degrees Celsius (minus 303 degrees Fahrenheit).

dark Photo: The Darkside-50 experiment consists of three chambers. An argon-filled tank is suspended inside a liquid-filled, room-sized steel sphere, which is  suspended in a three-story tank filled with ultrapure water. Researchers are looking for collisions of dark matter and visible matter in the  inner chamber, and the outer chambers help distinguish dark matter  particles from other particles. 


The active part of the detector, swathed in Teflon, holds 50  kilograms (about 110 pounds) of active argon — hence the name  DarkSide-50. At the top and bottom of the vessel are rows of  photodetectors that spot the light from the collisions. In addition,  copper coils collect electrons stripped from argon atoms by their  recoiling sibling — this helps in determining where within the detector  the collision occurred.

The argon-filled vessel sits inside a room-sized steel sphere that is suspended on stilts and filled with 7,000 gallons of a fluid called  scintillator. The sphere sits inside a three-story-high cylindrical tank filled with 250,000 gallons of ultrapure water.

Both of the outer chambers will help distinguish WIMPs from  cosmic-ray muons and particles called neutrons, which are emitted from  trace amounts of radioactivity in the materials used to construct the  detector.


Photo: The DarkSide-50 research team is made up of faculty, students and  researchers from dozens of institutions around the world, including,  from left, Luca Grandi, an assistant professor at the University of  Chicago, Richard Saldanha, an associate fellow at the University of  Chicago, and Hanguo Wang, a researcher at the University of  California-Los Angeles. (Photo by Yury Suvorov, University of  California-Los Angeles

A WIMP interacting with the argon will be seen nowhere else, whereas  the neutron and muon will interact with both the argon and either the  scintillator or water, allowing the scientists to distinguish WIMPs from the other particles.

Between DarkSide-50 and the roughly three dozen other detectors now  in operation or planned, many physicists think that dark matter  particles will be found within the next five to 10 years. However,  multiple experiments may be needed to confirm the sighting. A different  detector at Gran Sasso, called DAMA, has been reporting dark matter  particles since 1998, although the physics community has remained  skeptical. The Large Hadron Collider, currently offline for maintenance, will begin smashing protons again in 2015 and could create particles of dark matter.

"If DarkSide-50 finds dark matter, then we will have confirmed that  it is made of elementary particles, and it becomes something that we can study in a laboratory," Meyers said. "Since we can only detect about  three particles a year, we won't be bottling it any time soon. But  because we will know how to see it, we can start to study it."

"Whatever dark matter is, it is something new, something that has  never been detected before," said Taylor, who is now applying to  graduate schools and hopes to continue working on the search. "That is  just very exciting."



Source: Various online and offline sources.

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