Faculty Research: Exactly What is Dark Matter?
Assistant Professor of Physics James Battat Attempts to Answer One of the Most Perplexing Questions About the Universe.
By Mark Wolverton
Through a unique experiment based deep inside a New Mexico salt mine, Assistant Professor of Physics James Battat is attempting to resolve one of cosmology’s most profound and fundamental mysteries: the existence of dark matter.
Scientists hypothesize that about 80 percent of the matter in our universe consists of dark matter, made up of particles that are completely different from the familiar protons, neutrons, and electrons that make up regular matter (e.g., stars, planets, people). Regular matter has the almost universal ability to interact with light. But dark matter emits no telltale light or other electromagnetic radiation. How do you find something that is essentially invisible?
That’s the goal of the Dark Matter Time Projection Chamber (DMTPC) experiment, a collaboration among researchers from Bryn Mawr College, Massachusetts Institute of Technology, Boston University, Brandeis University, the University of Hawaii, and Royal Holloway, University of London. Battat’s work with the DMTPC project helped garner him a prestigious Sloan Fellowship—the only one awarded to a faculty member from a liberal arts college in 2012.
Battat’s research addresses a recent paradigm shift in fundamental physics—the particles we know about make up a tiny fraction of the total mass of the universe. Think of this as the ultimate Copernican Revolution. We long ago parted with the idea that the Earth is the center of the universe. Then we accepted that the sun is just an “average” star in our galaxy—which, we learned, is just one of billions of galaxies in the cosmos. We now know that all the matter that we have ever seen or touched is a minority contributor to the universe.
A leading dark matter candidate is the theorized weakly interacting massive particle, or WIMP. Since WIMPs by definition are “weakly interacting” with ordinary matter, their predicted properties assume that most pass through the Earth (indeed your body) unnoticed, but that once in a great while, a WIMP will score a direct hit on an atom, making its nucleus recoil in a characteristic and measurable way. However, since other subatomic background interactions can fake the tiny signals emitted by these collisions, a positive identification of WIMPs has eluded physicists.
The DMTPC project is trying to reconstruct the directionality of these collisions, key to distinguishing true WIMPs. Current theories, backed by computer simulations, hold that a sphere of WIMPs encompasses the familiar spiral arms of our galaxy and spreads far beyond them. As the sun and the rest of our solar system orbit around the galactic center, a “headwind” of WIMPs should be created from the direction of that movement, just as more raindrops strike a moving car’s windshield than its back window. Currently, the Earth’s galactic motion points toward the constellation Cygnus, so colliding particles coming from that direction would most likely indicate the presence of dark-matter WIMPs.
Battat and his colleagues currently use a prototype detector filled with low-pressure tetrafluoromethane gas to identify WIMP collisions and are constructing a more sensitive detector, about a cubic meter in size. Test runs of the detector were conducted at MIT before the instrument was placed in its permanent home, half a mile underground in the aforementioned salt mine. (The underground location shields the detector as much as possible from stray neutron and cosmic background radiation.) Results from the prototype’s first year of operation have been published, and the research group expects to improve on that search within the next year.
Bryn Mawr students, both undergraduate and graduate, will join Battat in the subterranean hunt for WIMPs. “DMTPC has benefitted from significant undergraduate contributions,” Battat says. “We’re definitely going to carry that tradition forward here at Bryn Mawr, and there are already undergrad students working on this project. I think it’s well suited for an undergraduate educational experience, be it thesis work or just getting your feet dirty in the lab.”
Or even in a salt mine below the New Mexico desert.