Next generation of dark matter

Craig Hogan

Craig Hogan, director of the Center for Particle Astrophysics, wrote this week’s column.

Scientists gathered at a Fermilab workshop last Friday to discuss the hunt for cosmic dark matter. They agreed that we may finally be closing in on a long-sought quarry. Fermilab theorist Dan Hooper succinctly expressed their sense of hopeful anticipation in the figure shown below. Technology is improving rapidly – faster than Moore’s law for computer speed – and theorists expect a discovery soon.

The Department of Energy and the National Science Foundation recently announced that they will competitively fund advanced dark-matter searches with Generation 2 detectors. Fermilab projects will be part of this new initiative, and one of them could be the first to detect this new form of matter.

Astronomers first discovered hints about eighty years ago that intergalactic space is filled with vast quantities of gravitating matter that emits no light. Over the years, they accumulated substantial evidence, from gravitational effects, that most of the mass of galaxies is invisible.

The favorite explanation is that this dark matter is actually an entirely new form of matter, left over from the early universe. That simple hypothesis explains a lot of precision measurements of cosmic structure. Another idea is that dark matter interacts weakly with regular matter, which would allow us to study it directly with very sensitive particle detectors.

Theory says that these weakly interacting massive particles (WIMPs), if they exist at all, would have been produced in about the right quantity in the hot Big Bang and would explain all the dark matter in galaxies today. The weak interaction is, well, weak, making dark-matter particles, like neutrinos, hard to detect. On the other hand, galaxies such as our own Milky Way are supposed to be filled with them, and we know what type of particle interaction we are looking for. As illustrated in Dan’s figure, we now finally have the technology to detect WIMPs, if they exist.

Fermilab is pursuing several experiments and technologies to detect galactic WIMPs directly. These need to be deployed deep underground to record the rare occasion when a WIMP zooming through our galaxy bounces off a nucleus in the apparatus. In addition, Fermilab is involved in experiments at the Large Hadron Collider, where scientists try to produce dark-matter particles in powerful proton-proton collisions.

Dan Hooper’s schematic plot shows how dark-matter experiments are becoming more sensitive to weaker and weaker particle interactions over time. If cosmic dark matter is made of Weakly Interacting Massive Particles, we should find them in the next decade.