Weighing the CHAMPions of the universe.

Weighing the CHAMPions of the universe

Roughly 90 percent of the universe's mass is invisible, making its presence felt only through the influence of gravity. The effort to identify the nature of this elusive material has spawned a host of dark-matter candidates--ranging from congregations of dim, cold stars to swarms of hypothetical elementary particles -- but there is no consensus yet on a leading contender. Now theorists have pushed a new, "seemingly outrageous" competitor into the fray: charged massive particles, or CHAMPs.

"People have been excluding the possibility of charged dark matter for no good reason and limiting themselves to neutral particles," says physicist Sheldon L. Glashow of Harvard University. "If you don't know what dark matter is, it would seem wise to be open-minded."

Glashow and his collaborators propose that dark matter consists of stable, very massive, electrically charged elementary particles left over from the Big Bang. These hypothetical particles would have masses between 20.000 and 1 million times the mass of a hydrogen atom. On Earth, such particles would be disguised as "preposterously heavy isotopes of known chemical elements," the researchers say.

For example, a positively charged CHAMP could attract an electron to become the equivalent of a neutral atom. "Aside from its enormous mass, it would have the chemistry of a hydrogen atom," Glashow says.

"It's the resuscitation of an idea that people had always dismissed out of hand," says R. Sekhar Chivukula of Boston University. "Everyone has always assumed that if dark matter were charged, we would have detected the extra stuff."

If CHAMPs exist, they would be a relatively rare constituent of the Earth, perhaps with a concentration similar to that of gold, Glashow says. It should be possible to design experiments to either detect these heavy particles or prove they can't exist. Previous, unsuccessful searches for heavy or anomalous isotopes don't necessarily exclude CHAMPs.

Eric B. Norman and his colleagues at the Lawrence Berkeley (Calif.) Laboratory have already begun looking for telltale traces of massive, negatively charged elementary particles trapped in iron and lead nuclei. So far, they have found no evidence for such particles, which would behave like extremely heavy electrons.

Glashow argues that Norman's group has looked in the wrong places. He recommends a search for "superheavy" hydrogen isotopes and suggests carbon nuclei as more likely hosts for negatively charged CHAMPs.

Moreover, it may be much easier to detect CHAMPs in lunar samples or chunks of meteorites. Because the moon has no atmosphere and little geological activity, CHAMPs flying through space would crash into its surface and presumably settle there in greater abundance than on Earth.

"We are planning to do some experiments to look for carbon nuclei having attached massive particles and possibly to look for superheavy hydrogen," Norman says. "We've got half a dozen meteorite samples, and we're pleading with NASA people to give us a moon rock."

Most astrophysicists believe that one dark-matter candidate will eventually turn out to be the dominant constituent of the universe. If CHAMPs are discovered and proved the missing matter, knocking rivals such as weakly interacting massive particles (WIMPs) out of the ring, then scientists will have a marvelous new material to play with and investigate.

"They would be a delight to material science," Glashow and his colleagues speculate. "Depending on their chemical identity, they could be used to create ultrahigh-density plastics or alloys." They would also make possible particle accelerators that would render the Superconducting Super Collider "impotent and obsolete."