UCLA collaborates on dark matter

Jennifer Lauren Lee
Published: Friday, April 8, 2005

Invisible, foreign and reclusive, the dark matter that courses quietly through the universe may finally be within grasp. With the construction of a new detector for dark matter, UCLA scientists and their international collaborators are participating in the beginnings of a potential revolution in physics. The detection of this mysterious material may hold the key to teaching scientists about the fundamental laws of the universe, said David Cline, UCLA professor of physics and astrophysics and a main initiator of the design and construction of the detector. Collaborators at Rutherford Laboratory near Oxford University in England are putting the final touches on the detector, which is due to start collecting data this September in an underground laboratory in Yorkshire, England. At the size of a small office refrigerator (about 2 meters tall), the ZEPLIN II detector is the largest dark matter detector in the world, Cline said.

Shining the light on dark matter “We call it dark because it’s not supposed to absorb or emit any electromagnetic waves (visible and invisible light), because if (it) did, you would see it,” said Hanguo Wang, an associate research scientist in the UCLA Department of Physics and Astronomy and collaborator with Cline on ZEPLIN II. Making up nearly 80 percent of our universe, dark matter (and its counterpart dark energy) are the dominant materials of the universe, Cline said. Its massive abundance has made it an important factor in the creation of galaxies, said Mark Morris, professor of physics and astronomy at UCLA. “We wouldn’t be here without dark matter,” Morris said. Morris explains that their abundance would have caused the dark matter particles to clump together early in the development of the universe, forming the seeds that allowed baryons – the matter composed of protons and neutrons which forms everything from stars and black holes to people and tuna fish – to clump together to form galaxies and other cosmic structures much faster than they would have been able to on their own. “They’ll do it – they’ll start gravitating toward each other ever so slowly. But we’d still be waiting for structures to form. We’d still be in the cosmic dark ages,” Morris said. Scientists have inferred the existence of dark matter for 70 years, when scientists noticed that stars were orbiting much faster around the centers of their respective galaxies than they should have, Wang said, adding that at those speeds, the stars should have been flung off into space like discuses from a discus thrower. Scientists realized, Wang said, that there must be some sort of invisible mass holding the galaxies together. “That’s the only way that we so far have of inferring that there’s dark matter,” Morris said. “It doesn’t absorb light; it doesn’t emit light.” Light and dark matter are ships passing in the night; they absolutely don’t know of each other, or see each other, or care about each other.” According to Morris, scientists still do not know what dark matter is, though they have inferred several of its properties indirectly. Dark matter is anything that does not emit light. It could technically, Morris said, be made from objects such as black holes or clouds of intergalactic gas, which are composed of protons and neutrons, but which do not emit light. But calculations of the amount of baryons in the universe since the Big Bang tell scientists that there is not enough of this matter floating around to account for these observations. “We don’t know what it is. It sort of leaves us philosophically in a hole. We’re a minor constituent of the universe – it’s kind of nice to know what’s going on around us,” Morris said.

Catching the elusive WIMP The particles that scientists have put forth as candidates for dark matter are composed of a completely foreign material. Named “WIMPs”– Weakly Interacting Massive Particles – these particles rarely interact with the stuff that humans and stars are made of, Wang said. As their name implies, Wang explained, WIMPs interact “weakly” with other particles, allowing them to pass through Earth millions of times without interacting with it. For a detector like ZEPLIN II, “that means maybe in 1000 days we get one event,” Wang said. To find this elusive matter, Cline, Wang and their colleagues from the United Kingdom, Russia and Portugal are using a detector filled with liquid xenon, which Cline said is ideally suited to dark matter detection. The xenon’s physical properties will allow scientists to distinguish between a reaction with a dark matter particle and a reaction with a known particle. When a dark matter particle collides with an atom of xenon, it will leave a “recoil” signature, Wang said, as when one billiard ball imparts energy to another. Another telltale sign of dark matter, which flows through the solar system in a single direction like a cosmic jet stream of particles, would be a signal that comes from one direction during half the year and the opposite direction during the other half. This regular annual variation should distinguish the dark matter from background noise, Cline said, allowing scientists to admit the new particle as a possible candidate for dark matter. “These are the key signatures that we’ve discovered dark matter, and not some random background or some mistake in our detector,” Cline said. They are also placing the detector 4,000 feet under the ground, in the Boulby salt mine in Yorkshire, England, in order to get rid of distracting background noise from cosmic rays that hit Earth from space. “This room has dark matter in it. ... It’s coursing through you and me now,” Cline said, referring to his office. “But you couldn’t detect dark matter in this room because there’s too much background (noise). So you have to go deep underground – thousands of meters underground,” Cline added.

A renaissance in physics? Cline said that the search for dark matter has become a worldwide effort within the past 15 years, with detectors on every continent. “If a dark matter particle could be detected, it would be the biggest revolution in particle physics since the electron, proton and neutron were discovered,” Morris said. “I think we’ll find it in our lifetime,” he added. “We know it’s there; how can we not find it eventually?” Cline said UCLA and other dark matter groups around the world could be very close to discovering dark matter. “It’s the most important question we need to answer today in physics,” Wang said. Cline added that the discovery of dark matter would be even more profound since it would tell scientists about the most dominant matter in the universe. “It would be a wonderful development in our field because it would be a renaissance in our field. It would be like going back a hundred years almost, to the time when people went up on mountains and did experiments.” Cline said. “So of all the fields of science right now that I know of, this is one of the most exciting fields because it may be on the cusp of a discovery.”