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The Hunt for Dark Matter

In a man-made cavern deep beneath a mountain, scientists are hoping to shed light on one of the most mysterious substances in the universe - dark matter.

The Gran Sasso National Laboratory seems more like a Bond villain's lair than a hub for world class physicists. It is buried under the highest peak of Italy's Gran Sasso mountain range in the Abruzzo region. The entrance is concealed behind a colossal steel door found halfway along a tunnel that cuts through the mountain. Though this may seem excessive there is a good reason for its subterranean location; the 4,600 feet of rock above means that it is shielded from the cosmic rays that constantly bombard earth’s surface. It provides scientists with the "silence" they need to understand some of the strangest phenomena known to physics.

Inside three vast halls, a raft of experiments are running, yet with their latest addition, DarkSide50, scientists are setting their sights on dark matter.

Everything that is known and seen in the universe only makes up about four percent of what is out there. The rest, scientists believe, comes in two enigmatic forms. They predict that about 73 percent of the universe is made up of dark energy, a pervasive energy field that acts as a sort of anti-gravity to stop the universe from contracting back in on itself. Researchers believe the other 23 percent comes in the form of dark matter. The problem is that nobody has seen it.

Dr. Chamkaur Ghag, a particle physicist from University College London, explained: "We think it is in the form of a particle. We have protons, neutrons and electrons and all these regular normal particles that you associate building things with. We think dark matter is a particle too, it's just an odd form of matter that we don't perceive it very readily, and that is because it doesn't feel the electromagnetic force - light doesn't bounce off it, we don't interact with it very strongly."

Physicists have called these dark matter contenders Weakly Interacting Massive Particles— or WIMPS. They believe millions of them are passing through us every second without a trace but occasionally one will bump into a piece of "regular" matter and that is what they are hoping to detect with DarkSide50.

Inside a house-sized tank, a large metal sphere holds a particle detector called a scintillator. This container is filled with liquid argon and a thick layer of the element in its gas form. "If a dark matter particle comes in and hits the argon the recoiling atom gets a kick of energy and it quickly tries to get rid of it," said Dr. Ghag. "In argon it gets rid of it by kicking out light; it sheds photons. And those electrons drift up into a gas layer and when they hit the gas you get another flash of light."

Until now, the hunt for dark matter has proved elusive.

Experiments have run for long periods of times without as much as a hint of dark matter. One, called XENON100, which is also in Gran Sasso, ran for the course of a year but only saw two "events"—not enough to rule out that this might have been some stray background radiation. But with DarkSide50 there seems to be a new push to find some answers.

Alongside this experiment, another large detector—LUX— which is in a gold mine in South Dakota in the United States will soon be coming online and in the next few years scientists are planning even more ambitious detectors such as XENON1T and LUX-Zeplin, hoping to find the first experimental evidence of these particles.

Aldo Ianni, from the DarkSide50 team said, "Dark matter is really a major scientific goal at the present time. It will help us understand a big fraction of our universe that we don't understand at the present time. We know there is dark matter but we have to understand what it is made of."

Professor Stefano Ragazzi, director of the Gran Sasso National Laboratory, hopes that the first glimpse of dark matter will be in his research facility. "The feeling is that dark matter could be just around the corner, so everybody is rushing to be the first to find it," he said. However, Ragazzi admits that there is always the chance that these experiments may find nothing at all—and dark matter may not be in the form of WIMPs. "We may find that we have the wrong hypothesis… dark matter may be something completely different. But it may be even more interesting not finding dark matter than finding it.”



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