Lightweight Dark Matter?

A team of physicists has proposed that the missing matter in the universe consists of a new kind of particle, one much less massive than the current favorite. Although the work is controversial, it shows how little is known about most of the mass in the cosmos.

Most physicists agree that about one-quarter of the stuff in the universe is "dark matter" made up of an exotic form of matter that hasn't been discovered yet. The leading candidate is known as a WIMP--a weakly interacting massive particle, such as the one suggested by supersymmetry theory, a popular extension to the Standard Model of Particle Physics.

In the most recent issue of Physical Review Letters, physicists present evidence that a different kind of particle entirely--at least 1000 times lighter than a WIMP--is responsible for dark matter. "It's really strange, contrary to everything assumed" about the dark-matter particle, says team member CÈline Boehm, a theorist at the University of Oxford, U.K.

The evidence is somewhat indirect for such a contrarian theory. The team's argument is based on observations of gamma rays emanating from the Milky Way made by the INTEGRAL satellite. If particles and antiparticles of light dark matter collide, they would produce electrons and antielectrons that in turn produce gamma rays with a specific energy. The abundance and sources of these gamma rays, as measured by INTEGRAL, match what would be predicted if the halo of dark matter around the Milky Way were made up of the lighter dark-matter particles.

But if such particles exist, they should have shown up in a particle accelerator by now, says John Ellis, a physicist at CERN, the European particle physics laboratory in Geneva. "I find it implausible that [a light] particle would escape detection," he says. Even Boehm admits that the case for a light dark-matter particle is tenuous. "I won't say I believe it, but I won't say I don't believe it either." The new paper does show, however, that until dark matter is actually found in the lab, it will remain one of the darkest mysteries of physics.

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Abstract of the Physical Review Letters paper

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