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The search for dark matter continues, more than a mile underground



For decades astrophysicists have been thinking about the strange movements of galaxies in the cosmos. The visible matter of the universe seems to be surrounded by an invisible counterpart, a material that interacts in no observable way with the surrounding matter, except for gravity: dark matter. Sophisticated measurements have led scientists to hypothesize that 85 percent of matter in the universe is dark matter, while only 15 percent are responsible for you, me, the planet, the stars, and everything else we can see.

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It is a satisfying explanation for our observations that has a big problem: a dark matter particle has never been directly detected. But the search for the elusive dark matter is close at hand to catch a shot, and instead of looking for evidence for the substance written in the stars, scientists are building ambitious experiments deep underground.

The SLAC National Accelerator Laboratory, which collaborates with Stanford University and several other laboratories and universities around the world, has just announced that the US Department of Energy is promoting and building a 6,800 foot underground dark experiment an old nickel mine has approved. The project, called the SuperCDMS SNOLAB experiment, will use supercooled silicon and germanium crystals to detect dark matter particles as they pass through our planet. It is expected that the experiment will be 50 times more sensitive than previous experiments, and it is planned to put it online in the early 2020s.

A SuperCDMS SNOLAB detector made of silicon and germanium crystals and manufactured at Texas A & M University

Matt Cherry / SuperCDMS Collaboration / SLAC National Accelerator Laboratory

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The DOE's Office of Science just approved $ 19 million to fund the project, which also gets $ 12 million from the National Scientific Foundation and $ 3 million from the Canada Foundation for Innovation. The experiment is the latest in a series of revised attempts to find dark matter, including another multi-million dollar project in an abandoned gold mine in South Dakota with a tank of 10 tons of liquid xenon as a dark matter detector. Several smaller experiments have attempted to detect dark matter, but so far none has been fruitful.

Theoretically, dark matter particles should be everywhere and fly freely through normal matter without anyone being wiser. The attraction of dark matter is remarkable in the behavior of galaxies, but the particles themselves are virtually unnoticeable (if they actually exist). The hypothetical building blocks of dark matter are called WIMPs or weakly interacting massive particles.

Paul Brink of SLAC works on a SuperCDMS SNOLAB engineering tower.

Chris Smith / SLAC National Accelerator Laboratory

"Our experiment will be the most sensitive for relatively light WIMPs – in a mass range of a fraction of the proton," says Richard Partridge, head of the SuperCDMS group at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), in a press release. "This unprecedented sensitivity will provide exciting opportunities to explore new terrain in dark matter exploration."

The SuperCDMS, or Super Cryogenic Dark Matter Search, will be held at the SNOLAB facilities near the city of Sudbury, Ontario. Scientists used the facilities that were in an ancient nickel mine more than a mile underground to study neutrinos in the past. In the underground mine, the experiment is protected from interference from charged cosmic particles that hit Earth from space.

It is believed that WIMP's ordinary matter without significant impact will pass the time. Occasionally, however, a dark matter particle should collide with a particle of visible material. These effects, when they occur, are unbelievably difficult to detect because they happen so seldom and are generally drowned out by the cacophony of other atomic collisions.

In the latest attempt to capture a particle of dark matter in action, physicists will use silicon and germanium crystals that have cooled to minus 459.6 degrees Fahrenheit, only a fraction of a degree above absolute zero. At absolute zero – which we can not fully reach – the particles stop moving completely. It is the coldest thing that matter can be. Near this state, every movement should be detectable at the atomic level.

The crystals, each resembling an oversized hockey puck, are arranged in four detector towers. Each tower will contain six crystals that will be put into their own units, and the four towers will be placed in a cryogenic container called SNOBOX to bring the experiment to near absolute zero. The SNOBOX and all four towers will live underground in the mine, waiting for dark matter to come along and give the crystals a nudge.

At the heart of the SuperCDMS SNOLAB experiment are four detector towers (left) with six detector packages each. The towers are mounted in the SNOBOX (right), a vessel in which the detector packages are cooled to near absolute zero.

Greg Stewart / SLAC National Accelerator Laboratory

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"The detector towers are the most technologically sophisticated part of the experiment, pushing the boundaries of our understanding of cryogenic devices and superconducting readings Berkeley

When a WIMP shows itself by hitting a particle in the crystal detectors, a series of tiny vibrations will resonate through the substance. The collisions should also produce pairs of electrons that cause electron deficiency, which would amplify the atomic vibrations in the material.

When a WIMP (white trace) hits an atom within the detector crystals of the experiment (gray), the crystal lattice will vibrate (blue). The collision will also send electrons (red) through the crystal, which amplify the vibrations.

Greg Stewart / SLAC National Accelerator Laboratory

To prepare for the experiment, the SLAC Nuclear Accelerator Laboratory built and tested a prototype detector. "These tests were an important demonstration that we were able to build the actual detector with high enough energy resolution and detector electronics with enough noise to meet our research goals," said Paul Brink of KIPAC, who has completed the detector fabrication at Stanford

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The first detector tower could be delivered to SNOLAB by the end of the year, and after the experiment there is nothing to do but wait for a WIMP when a particle of dark matter hits one of the crystal detectors and When it rings like an atomic bell, astronomers and physicists will finally have strong experimental evidence that dark matter is indeed the cause of strange movements in the sky.

When scientists hear silence, the hunt for dark matter and an explanation for the behavior of the universe will continue.

Source: SLAC National Accelerator Laboratory [19659030]
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