From Outer Space to Bowdoin Labs: The Hunt for Dark Matter

Story posted October 26, 2007

Deep under an Italian mountain, scientists are attempting to prove the existence of "dark matter," particles so plentiful they are believed to account for 90 percent of the universe's mass, but so elusive that they remain undetectable. Deep in the basement of Searles Science Building, Jimmy Lindsay '09 is trying to help them find it.

Working with Bowdoin faculty, Lindsay, a junior from Portland, Oregon, assists in the international search for "dark matter."

Under the guidance of Madeleine Msall, associate professor of physics and astronomy, Lindsay is charting how calcium tungstate crystals react to energy deposited by a highly focused laser beam. This information will provide a baseline to demonstrate how the crystal might react to dark matter particles in the international project called CRESST (Cryogenic Rare Event Search with Superconducting Thermometers).

It is the continuation of work Lindsay began this summer under a Maine Space Grant Consortium Fellowship from NASA.

NASA has established affiliates in every state as a form of public outreach. A major component of that presence is funding space-related research projects for qualified graduate and undergraduate institutions. Lindsay is one of six Bowdoin students awarded a NASA fellowship this year.

"They are thinking about the next generation of scientists," said Thomas Baumgarte, associate professor of physics and astronomy and Bowdoin's liaison with the Maine Space Grant Consortium.

Physicists and astronomers can determine the weight of the universe by tracking stars, because stellar orbits are related to the mass of all the objects within the orbit, but a large discrepancy exists between this calculated mass and what astronomers have observed firsthand. The material making up the undetected mass is collectively known as either "dark matter," because it emits no light, or WIMPs (Weakly Interacting Massive Particles). Scientists used to theorize that dark matter was located in a single concentration of black holes, but none has been detected. They now believe that this dark matter is ubiquitous, permeating our entire galaxy and forming a halo around it.

Scientists from 20 countries are working to detect these particles in a lab nearly a mile under the Gran Sasso in central Italy's Apennine Mountains. The solid limestone above the lab shields their cryodetectors from atmospheric interference.

"These are high-energy particles whizzing in from outer space," Msall said. "The usual detectors don't pick them up, but we think they're out there. The challenge is, we're making a new detector for a new particle we have no information on yet. Jimmy and I are working on the calibration of the detector."

Lindsay, a double history and physics major, began his research this summer with a crash course in solid-state physics, Msall's specialty. He also spent a week with Rachel Beane, associate professor of geology, using a scanning electron microscope to model the orientation of atoms in the thinly sliced, one-inch square of calcium tungstate crystal that was to be the target of his laser beam.

The purpose of his experiment was to see how the atoms in the crystal react to energy. In order to do that, the crystal had to be frozen in liquid helium to 2 degrees Kelvin, then exposed to a laser beam that skims the surface in a precise grid pattern.

"Atoms jitter from natural heat," Lindsay said. "We needed to freeze the crystal to make the atoms be still. The laser then excites localized areas of the crystal."

When the laser strikes the crystal, the photons (the smallest unit of electromagnetic energy) travel through the crystal and strike the atoms, generating vibrational energy (phonons). The atoms warm as they vibrate. By measuring the changes in temperature the laser causes, Lindsay can map how calcium tungstate reacts to stimuli, laying the groundwork for being able to determine if dark matter is reacting with it.

"We are hoping the crystal will react with dark matter, if anything," Lindsay said. "We need a clear understanding of how energy disperses in it."

Sounds simple enough. But each step of Lindsay's research provided him with a new skill to learn and new challenges to overcome.

It's hard to make a thermometer that can measure changes in temperatures that low, so Lindsay fashioned his own "bolometer," which works on the same principal as a digital thermometer.

To do that, he had to learn how to coat a portion of the crystal with an evaporative film of aluminum. He then connected wires to the aluminum and ran a current through it. Resistance is proportional to temperature, so by measuring changes in the resistance, he could calculate relative changes in temperature.

At the same time he was learning this process, Lindsay was programming the computer that operates the laser so that it would sweep across the crystal in exactly the pattern he needed.

By the end of the summer, he had not gotten conclusive results because the laser was unable to emit its energy in a concentrated enough burst. "It's the difference between gently pushing (on the atoms) and thumping on them," Msall explained. A better laser should be available by late fall.

Lindsay is pursuing the research this semester and might expand it into an honors project.

"There's plenty of work to continue next summer," he said.
