In The Search For Dark Matter

Tuesday, November 11, 2008

The Exotic Particles Would Explain a Lot About the Universe, and That Promise Has Scientists Going Underground.

You might think an astrophysicist would spend much of his time with his head in the stars. Instead, Sean Paling often squeezes into a cage with a bunch of burly miners and travels for six minutes in darkness to the bottom of Britain's deepest working mine.

At a lab here, 3,300 feet underground, Dr. Paling is searching for one of the most elusive objects in the universe: a wimp, or weakly interacting massive particle. Wimps are leading candidates for dark matter, which is believed to account for up to 95% of the mass of the universe. Something that big would be easy to spot except for the fact that dark matter is invisible. That doesn't stop the elusive mass from making its presence felt by the immense gravitational tug it exerts on stars, galaxies and other cosmic bodies.

"For 20 years, the miners have been asking if I've found it yet, and for 20 years I've been saying no," says Dr. Paling, an astro-particle physicist at England's Sheffield University, who has been searching for wimps at Boulby since 1989. "You can understand their confusion."

Unraveling the secret of dark matter is one of the grandest prizes of astrophysics because it is the key to understanding the shape, size and even the fate of the universe. Knowing how much dark matter there is will tell us whether the universe will keep expanding, or expand to a point and then collapse, or get bigger and bigger and then stop. More parochially, it can help us predict how Earth's neighborhood, the Milky Way galaxy, formed and how it might evolve.

But it is a difficult quest. Wimps rarely interact with normal matter such as atoms; indeed, billions of wimps may be darting right through the Earth every second without hitting anything. Detectors must be installed deep underground because on the surface, the profusion of other cosmic rays would crowd out a wimp's signal, which is feeble, because wimps move relatively slowly.

Wimp hunts have been going on for years in the U.K., Italy, Spain and France, as well as at a disused iron mine in Minnesota. The race intensified in April, when scientists working beneath Italy's Gran Sasso mountain announced that they had found signals of dark matter streaming in from space, though the results are in dispute.

The newest competitor on the scene is the European Organization for Nuclear Research, or CERN, the group that runs Europe's new Large Hadron Collider outside Geneva. CERN scientists hope to find evidence of dark matter in a different way, by smashing together subatomic particles at high speed and seeing if any wimps emerge.

Which group will get there first? "It depends on who nature is kind to," says Tom Le Compte, a particle physicist at CERN and Argonne National Laboratory in Argonne, Ill.

The U.K. project is unusual because it is based in a working mine, a tough environment for an experiment that relies on minute measurements and ultra-clean, ultra-sensitive equipment. The 35-year-old potash mine, near England's northeast coast, has more than 620 miles of dark and dusty tunnels, including some that delve under the North Sea.

"Unlike the miners," says Dr. Paling, "we are ever so delicate."

Dr. Paling and his colleagues first set up an extremely basic wimp detector at the mine. In 2003, a £2 million ($3.1 million) investment got them a full-scale lab with far more sensitive machines. It is run by Sheffield University, Rutherford Appleton Laboratory and other British and international groups.
One recent morning, Dr. Paling donned miner's gear -- overalls, boots, helmet, lamp and respirator -- and took the ride down the shaft. As he and his colleagues walked to the lab, a grimy, salt-encrusted vehicle went rattling by, carrying a group of miners to an underground excavation area 30 minutes away.

Dr. Paling pointed upward and said: "You can't feel it, but only one-millionth of the cosmic rays hit you here, compared to on the surface. We expect some are wimps."

The dark-matter lab is a long, narrow structure, suspended by cables inside a cavern. There are places in the mine where the temperature reaches 111 degrees Fahrenheit. "It's because you're closer to hell," jokes David Pybus, a spokesman for Cleveland Potash Ltd., which operates the mine.

That day, in a tunnel far from the lab, a loud, remote-controlled excavator made a racket as it gnawed through the walls of a tunnel, spewing salt everywhere. To keep out such contaminants, the lab is protected by a series of doorways and air blowers. Also vital to the operation is a local "cleaning lady" with a mop and bucket.

Most wimp detectors contain a target material, typically a liquid or a solid that is particularly sensitive; one of the detectors at Boulby uses carbon disulfide gas. The hope is that a wimp of cosmic origin will fly through the surrounding rock and, if the scientists get really lucky, strike a particle of the target material. By studying the collision, a computer can tell whether the particle is a wimp or something else.

That is the challenge. Although the walls of the mine give off only very low amounts of radiation, the detector picks up the signals of alpha, gamma and other forms of radiation emitted by materials in the lab, including the detector itself. Dr. Paling and his colleagues struggle to keep these background effects to a minimum.

One of the current Boulby detectors is called Drift II, for directional recoil identification from tracks. It is specially designed to tap into something called the wimp wind. As the earth rotates on its axis and also zooms around the sun, there are times when a stream of wimps would be expected to come straight at you -- in your face, as it were -- and other times when the wind would be at your back. If scientists can detect such a modulation, it would be extra evidence for wimps, since such a directionally changing signal can't be mimicked by background sources.

On a computer screen, Dr. Paling watched as the detector registered a series of particle collisions. The first he dismissed as an alpha particle, based on the length of its track. Another turned out to be a gamma particle. No wimps today. Dr. Paling doesn't usually study each collision in real time, but inspects a record compiled by the computer.

If scientists armed with sufficiently sensitive detectors fully explore the range of possible wimp interactions with matter and still don't find the elusive particles, it would mean that wimps may not exist -- and some basic observations about the universe would have to be re-examined.

"A little doubt starts to occur in your mind now and again," Dr. Paling says. "Then you look at a galaxy that's rotating 10 times faster than is possible given the missing mass, and you know the wimps are out there."