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DarkSide-50 searches for dark matter from deep under an Italian mountain

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January 22, 2014

The spherical middle chamber of DarkSide-50 is filled with 7,000 gal of scintillator fluid

The spherical middle chamber of DarkSide-50 is filled with 7,000 gal of scintillator fluid

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Like the Higgs Boson, dark matter is one of those things in the Universe that evidence points to, but is very difficult to pin down. A team of researchers is looking to verify the existence of this most elusive of ingredients that is thought to make up 23 percent of the Universe using powerful detectors buried deep in an Italian mountain.

Unless our current theories of gravity are wrong (which some scientists think is a possibility), scientists say there must be some yet undiscovered matter to explain why the galaxies haven't flown apart. Dark matter is proposed as filling this gap by creating the gravity that keeps the Universe, as we know it, together.

The DarkSide-50 project is an international collaboration between Italian, French, Polish, Ukrainian, Russian, and Chinese institutions, as well as 17 American universities, which aims to pin down dark matter particles, which are thought to make up 23 percent of the Universe. The project team spent last (Northern hemisphere) summer assembling the detector in a laboratory deep within the Gran Sasso mountain, which is accessed via an exit off a six-mile (9.6 km) long highway tunnel in Italy.

Measuring in at about the size of a grocery bag, the detector contains 10 gallons (38 liters) of pure argon that has been cooled to -302.8° F (-186° C), putting the element in liquid form. According to the researchers, the active, Teflon-coated part of the detector holds 50 kg (110 lb) of argon, which provides the 50 in the experiment’s name. Rows of photodetectors line the top and bottom of the device, while copper coils collect the stripped electrons to help determine the location of collisions between dark matter and visible matter.

The research team, as well as many other scientists, believe that a particle known as a WIMP (weakly interacting massive particle) is the prime candidate for dark matter. WIMP particles have little interaction with their surroundings, so the researchers are hoping to catch one of these particles in the act of drifting aloof. Scientists believe these particles can be detected when one of these particles collides with the nucleus of an atom, such as argon.

Only a few particles will be detected each year, so great care must be taken to keep the e...

To aid in the collision and detection process, the researchers have constructed the DarkSide-50 experiment. By cramming the chamber with argon atoms, the team increases their chance of seeing a collision – though they are still a rare occurrence, with just a few detected each year. The recoil from these collisions can be seen in a short-lived trail of light. Using photodetectors, the researchers can see these excited argon atoms and potentially separate the WIMP reactions from background events. This is why the experiment is being conducted deep in the Italian mountain of Gran Sasso, so as to remove as much of the background as possible.

To aid in filtering out background events, the detector sits within a steel sphere that is suspended on stilts and filled with 7,000 gal (26,500 L) of a fluid called scintillator. The sphere itself sits inside a three-story-high cylindrical tank filled with 250,000 gal (946,350 L) of ultrapure water. These different chambers help the researchers differentiate WIMP particles from neutrons and cosmic-ray muons, which are emitted from trace amounts of radioactivity by the detector’s construction materials.

The weeks-long process of filling these chambers was completed in the (Northern Hemisphere) summer of 2013. After final checks were completed in the following autumn, the researchers began collecting data. Later this year, the team plans to increase sensitivity by replacing the current argon (from the air) with underground argon from Colorado.

The DarkSide-50 experiment is similar to and draws on Princeton’s experience with the detection of solar neutrinos – an experiment that was also conducted in the Gran Sasso mountain. Utilizing this experience, the researchers plan to increase the size of the DarkSide experiment, using 3 tons (2.7 tonnes) of argon rather than the 50 kg currently used, to increase the likelihood of detection. This will also require the detector to increase in size from the size of a grocery bag, to as Princeton University professor of physics, Peter Meyers, says, "the size of a Chevy Suburban."

The DarkSide-50 is one of about three dozen detectors on the hunt for dark matter, which leads many physicists to believe that elusive dark matter particles will be discovered in the next decade.

The video below is a Google Hangout with Peter Meyers, one of the lead researchers on the project.

Source: Princeton University

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