Evidence of things unseen
Dark matter makes up 85 percent of the material in our universe. It envelops our galaxy—yet scientists have never seen it. That's why physicist Betty Young is looking—right here on Earth.
Betty Young is claustrophobic. Being inside an MRI machine feels “icky” for her, and one of her childhood nightmares was getting buried alive.
But in February 2002, she traveled to a mine near Ely, Minn., squeezed into a cage with about 20 other people, and descended through a pitch-black shaft to a cavern nearly half a mile underground.
Looking for dark matter: physicist Betty Young. Photo by Charles Barry.
“It was so exciting,” says Young, a professor of physics at SCU.
Why would she want to do such a thing? Young is part of a worldwide effort to hunt for dark matter, the enigmatic substance that composes most of our universe’s matter. Physicists have never seen dark matter directly—yet they believe dark matter exists because, for starters, they can see its gravitational effects on the speeds of galaxies and stars. Scientists can also detect dark matter indirectly by watching how light from faraway galaxies bends around it. In March, for example, researchers reported observations with the Hubble Space Telescope showing a core of dark matter that remained from a collision of galaxy clusters more than 2 billion light years away. But scientists are not quite sure what dark matter is made of. Their best guess is that it consists primarily of particles called weakly interacting massive particles, or WIMPs. Young and her colleagues are searching for these elusive WIMPs—and to screen out the background noise of everyday particles, they must house their ambitious experiment in the Soudan Underground Laboratory, a 2,341-foot-deep scientific facility in an old mine.
Young’s contribution to the project is her expertise in detectors, incredibly sensitive instruments that can pick up the tiniest effects of incoming particles. Building and running these detectors demands the use of ultra-clean fabrication techniques similar to those employed by high-tech computer chip manufacturers. It also requires cooling semiconductor materials to near-absolute zero—the equivalent of about -460 degrees Fahrenheit—and coaxing information from minuscule sensors. “I’m doing the ultimate in benchtop physics,” she says.
Hooked on physics
When she was a kid, Young seemed like a prototypical physicist-in-training. She made a scale model of the solar system, built a small replica of the Apollo spacecraft, and took an extra science program after school. But she didn’t consider science a potential career. “Girls weren’t encouraged to take any science and math classes,” she says. “I thought it was something I could do at home.” So Young focused on music in school, playing percussion instruments such as drums, timpani, and marimba.
But when she took physics as an undergraduate at San Francisco State University, “that was it,” she says. “I just got involved with physics night and day and nonstop.” Young worked with physicist Roger Bland—“the MacGyver of laboratory physics”—on an experiment to search for particles called quarks in ancient samples of mercury and water.
Young then worked in research labs at Stanford University and the University of California, Berkeley, helping develop detectors that could sense elusive dark matter particles. These detectors relied on semiconducting materials such as silicon and germanium, the same materials used for computer chips. In an ultraclean basement fabrication facility at Stanford, Young—dressed head to toe in a white “bunny suit” to prevent contamination—was trained by chip industry experts to deposit thin layers of metal on a silicon wafer and etch features less than a hundredth of a millimeter wide.
The missing piece
Today, similar technology powers the Cryogenic Dark Matter Search (CDMS), a collaboration of about 75 U.S. and international experts on the hunt for dark matter. Young helped develop, test, and run the detectors, bringing her years of experience to bear on a tantalizing problem in physics. “She’s been one of our stars on that,” says Dan Bauer, who is based at Fermilab in Batavia, Illinois, and is the CDMS project manager.
The idea of dark matter goes back about 80 years. In the 1930s, astronomer Fritz Zwicky discovered that galaxies in a cluster were moving faster than expected, based on the amount of mass the cluster was thought to contain. He suggested that some unknown, additional mass was contributing to the effect. Later, other scientists found that stars on the outskirts of galaxies were also moving too quickly to be accounted for by the known mass in each galaxy.
“We find that more and more of it just doesn’t quite add up,” says Young. “So there’s some missing piece.”
Scientists called this missing piece “dark matter”—matter that does not emit or absorb light and thus can’t be seen with standard observing techniques. Astonishingly, this unseen material makes up about 85 percent of the matter in our universe. It envelops our entire galaxy, and the Earth is passing through it. “It’s going through us all the time,” says Young. “The hypothesis is that if dark matter exists, we are certainly in the wind of dark matter.”
When asked why this work matters, she answers, "It's sort of like asking Galileo, well, why bother using a telescope to look out there?"
The next question was: What was it made of? No known particle fit the bill. Therefore, it had to be a particle never seen before. The particle must have no charge, since matter that includes charged particles would emit or absorb light. It must interact with normal matter very rarely. And some scientists predicted that each particle has a relatively high mass, compared to other particles.
So scientists called this particle a “weakly interacting massive particle,” or WIMP. But the nature of the WIMP presents a problem. If the particle almost never interacts with normal matter, then detecting it is a formidable challenge.
Detecting a signal
That’s where the detector technology that Young helped develop comes in. The basic premise is this: Make a roughly hockey puck shaped piece of material, say germanium. A WIMP will almost never hit an atom in the germanium puck. But once in a very long while—perhaps every few years or so—it will.
When a WIMP hits an atom, that collision sends vibrations through the germanium crystal. Those vibrations travel to the crystal’s surface. If scientists can detect the vibrations, they know a particle has hit the detector.
But there’s a problem: At normal temperatures, the atoms in the crystal are already vibrating. So it’s virtually impossible to detect “new” vibrations amidst the existing chaos. “Everything is rattling,” says Young. “We would never see our signal. It would be lost in the noise of everyday life.” Imagine a bunch of tennis balls held together in a lattice by springs, all wobbling around. If you poke one of the tennis balls, it will wobble, but there’s no way to distinguish that wobbling from everything else.
The solution is to cool the detector close to absolute zero, the temperature at which essentially all atoms stop moving. That way, new vibrations from a particle collision will stand out.
Scientists need incredibly sensitive tools to pick up these vibrations. They know the vibrations will heat up the crystal by a tiny amount. So each hockey puck shaped detector has minute tungsten sensors patterned on its surface that detect changes in temperature. “They basically act like thermometers,” says Young.
These are no ordinary thermometers. If tungsten is cold enough, it acts as a superconductor, meaning that electrical current will pass through it with no resistance. But if the tungsten is heated by even a couple thousandths of a degree Fahrenheit, its resistance will shoot up. Researchers exploit this property by running a current through the sensor. If the resistance suddenly jumps, they know that the crystal has just gotten warmer.
On the cusp
Of course, WIMPs won’t be the only particles hitting the detector. If a detector is extremely sensitive, “it’s sensitive to everything,” says Matt Cherry ’07, a physical sciences research assistant at Stanford who is part of the CDMS team. “You can’t tell it to ignore something.”
To figure out if the collision came from a WIMP or another particle, researchers also measure a second effect of the collision. The collision will jolt electrons out of their normal spots, leaving free electrons and positively charged “holes” where the electrons used to be. Scientists can measure this charge signal. Together, the heat and charge measurements give researchers enough information to distinguish WIMPs from most other particles.
There is, however, one particle that is especially tough to tell apart from a WIMP: the neutron. Like WIMPs, neutrons also have a relatively high mass and no charge. And neutrons are everywhere. When cosmic rays from space slam into the atmosphere, they produce huge showers of particles, including neutrons. The best way to avoid them is to go deep underground. “Our worst possible nightmare would be to be in an environment where we had an uncontrollable or unknowable neutron background,” says Young.
|The detector: size of a hockey puck, and a tool to help answer the question, "What is everything?" Though they tend to come without an image of hundreds of billions of stars superimposed on them.
In 2003, stacks of these super-cooled detectors began running in a cavern in the Soudan Mine, as part of the CDMS II project phase. In late 2009, the team reported it had picked up two particle collisions that matched what they would expect for WIMPs. But because the team also expected about one spurious event caused by radioactivity, the evidence wasn’t convincing enough to declare victory.
Last year, the team installed bigger detectors as part of the next phase of the project, called SuperCDMS Soudan. Astrid Tomada ’01, M.S. ’06, a sensor engineer at SLAC National Accelerator Laboratory in Menlo Park who works on CDMS, compares the detector to a butterfly net: The bigger the net, the better your chances of catching a WIMP. And the team hopes to install even bigger detectors in the underground SNOLAB facility in Canada, which is three times deeper. “It’s deep enough that the rock actually starts to feel warm,” says Fermilab’s Bauer.
Young is unfazed that the team hasn’t seen a definite result so far. CDMS II still provided useful information: If an experiment doesn’t catch any WIMPs, researchers can narrow down how frequently a WIMP should interact with normal matter. Those results then allow scientists to rule out certain theoretical models of WIMPs. The team is now looking in the “sweet spot” of remaining models’ predictions, says Young. “It does feel like we’re on the cusp,” she says. “It really could happen.”
If that sounds too optimistic, rest assured that Young isn’t the type of person to chase after nothing. “You could say, well, it could just go on forever,” she says of the dark matter search. “I, for one, wouldn’t be interested.” CDMS isn’t the only group on the hunt; there are about a dozen other experiments in various stages of operation all over the world attempting to directly detect WIMPs colliding with ordinary matter. Other scientists are searching for evidence of WIMPs annihilating each other in space, while researchers at the Large Hadron Collider, a particle accelerator at the border of Switzerland and France, hope to create WIMPs by smashing other particles together.)
At home in the lab
While Young is intrigued by dark matter, she becomes the most animated when talking about her true passion: the lab. Many physicists spend most of their time at the computer running analyses, but Young is not one of them. “What really hooks me is pushing the envelope on technology,” she says. In one of her labs on the Mission Campus, a small, high-ceilinged room in the Daly Science Center, Young surveys the instruments, wires, and tools with the air of an experienced carpenter. She pulls herself up a short stepladder and sits on the top rung, feet tucked under her. “The astrophysics stuff is neat, but this is my bread and butter,” she says.
The National Science Foundation has provided strong support for Young's work at Santa Clara. Young, in turn, has passed down her enthusiasm for the lab to many students. Tomada and Cherry, whom Young advised during their undergraduate days at SCU, now help fabricate the CDMS detectors. And Young tries to instill experimental skills in her current undergraduates. She is derisive of programs that rely on “dry” labs, where computers simulate the experiment for students. “Forget it!” she says. “I mean, what is that? That is not teaching a person to be an experimental physicist. It’s not. It’s just ridiculous.”
Young got to work at the ultimate lab bench when she did shifts at the Soudan mine, helping to set up the cryogenic equipment and run the detectors. “I think that’s one of the things I will never forget in physics,” she says. “It’s spectacular. And it’s almost like walking into a futuristic world.” But, she says, “it feels like home because it’s just got physics stuff in it.”
When asked why this work matters, she answers, “It’s sort of like asking Galileo, well, why bother using a telescope to look out there?” That’s the role of science, says Young. “And that’s what physics does, fundamentally. What are we made of? What are we? What is everything?”
For Young, the fact that her expertise on detectors can be applied to one of the most fundamental problems in science today is “almost too good to be true,” she says. “It’s the ultimate physicist high, because you know what you’re doing matters.”
Roberta Kwok has covered science for Nature, Salon.com, Conservation, and others. She is the recipient of the American Geophysical Union’s Walter Sullivan Award for Excellence in Science Journalism. This is her first feature for SCM.
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