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Dark Matter

For thousands of years, people have gazed at the stars and wondered, "What's out there?" But it wasn't until the 1930s that anyone realized there is much more out there than meets the eye. Recent observations have proven that about 22 percent of the universe is made out of "dark matter" that is like the wind—although invisible, its influence can be seen. In a recent paper, SLAC theoretical physics professor Michael Peskin provides an overview of how scientists are trying to unlock the mysteries of dark matter.

Though there are many theories regarding dark matter, one of the popular claims suggests that dark energy is a Weakly Interacting Massive Particle (WIMP). These hypothetical particles rarely interact with visible matter, making them extremely difficult to detect outside of a particle accelerator. WIMPs have not yet been seen in particle accelerators, but that could soon change.

"Scientists have been working on dark matter questions for a long time," said Peskin. "But now the work is all coming to a head."

Scientists are using three types of experiments in their attempt to identify dark matter: particle accelerators, satellite telescopes and sensitive underground detectors.

At sufficiently high energies, particle accelerators may be capable of creating dark matter. The Large Hadron Collider (LHC) is scheduled for completion at CERN in Geneva, Switzerland in the spring of 2008, and will smash protons together at energies well above those expected to create WIMPs. And if WIMPs exist and are created by the LHC, detecting them will not be the issue. The real challenge will be proving that any newly discovered WIMPs actually are dark matter, considering that no current detection method has yet seen it.

Although detecting dark matter is difficult outside of an accelerator, any WIMPs created at the LHC will be readily apparent due to missing momentum in collision events. When particles are smashed together they create new particles that shoot out in many directions. These events must be balanced—the momentum of particles shooting out in one direction must equal the momentum of the particles shooting out in the opposite direction. An imbalance may imply the creation of a particle that scientists can not see.

Scientists expect many new particles to be created by the LHC and any WIMPs that are created may not actually be dark matter. To prove they are the missing mass, properties of new particles must be compared to the properties of dark matter observed outside of an accelerator.

One current type of experiment designed to determine dark matter's properties involves searching for WIMP annihilations throughout our galaxy. Theoretically, when two WIMPs collide, the resulting event creates quarks, electrons and photons that can be detected. One type of particle that is created, gamma rays, is the subject of a future search using the Gamma-ray Large Area Space Telescope (GLAST), scheduled for launch early next year. If a large, stable source of gamma rays were detected from an invisible source—not from stars or gases—it would imply a WIMP annihilation. Additionally, other experiments are searching for WIMP annihilations through positron and antiproton emissions.

Another type of search is taking place underground. Large, extremely sensitive detectors shielded deep beneath the earth's surface are searching for evidence of dark matter striking ordinary matter. One such experiment is the Cryogenic Dark Matter Search (CDMS) located in the 2,400-foot deep Soudan Underground Laboratory in Minnesota. These types of experiments are shielded by layers of rock that block cosmic rays, which otherwise produce events that could be mistaken for dark matter.

Results from LHC, compared to results from CDMS, GLAST, and other similar experiments, will let scientists begin the process of checking whether the particles identified at accelerators are the same as those that make up most of the mass in the universe.

"I wish it were five years from now and we had the answers," said Peskin. "But we don't have long to wait now."

—Ken Kingery, SLAC Today, August 8, 2007