These particles, called superweakly interacting massive particles, or superWIMPs, may constitute the invisible matter that makes up as much as one-quarter of the universe's mass.

UCI physicists Jonathan Feng, Arvind Rajaraman and Fumihiro Takayama report that these new superWIMPs have radically different properties from weakly interacting massive particles (WIMPs), which many researchers have long looked to as the leading dark matter candidate. The study was posted July 3 on the online version of Physical Review Letters.

The identity of dark matter is one of the most puzzling problems for those who study the nature of our cosmos. While as much as a quarter of the universe is made of this invisible mass, which plays a vital role in the structure of the universe, almost nothing is known about its composition. It is believed to be the celestial glue that holds galaxies together in their distinctive spiral shapes.

To identify this elusive dark matter, many astrophysical researchers have turned to WIMPs. These particles emit no light and are very difficult to detect. However, as their name suggests, they do have weak-force interactions with other particles, and they are expected to leave visible traces in experiments. Currently, research groups throughout the world are searching for WIMPs, so far without success.

But in studying theories that predict WIMP dark matter, Feng and his colleagues found that in many of these theories WIMPs do not live forever. According to Feng, many theorists have assumed WIMPs to be the lightest particles and thus the most stable. "But we've found that WIMPs are often not stable at all, because they can decay into lighter particles," Feng said, "and, all of a sudden, the WIMPs disappear."

These new, lighter particles are superWIMPs. Like their progenitors, they emit no light and have both mass and gravitational force. But they are incapable of the type of weak-force interactions that WIMPs have; they can only interact gravitationally. Since the gravitational force is not as strong as the weak force, these interactions are, as Feng calls them, "superweak." In turn, these particles will rarely, if ever, collide with other particles.

And, unlike WIMPs, superWIMPs are incapable of decaying into other particles. "They are absolutely stable," Feng said. "And because of this, they are a completely different, but perfectly viable, alternative for dark matter."

Like WIMPs, superWIMPs only exist theoretically. In fact, because superWIMPs do not have weak-force interactions, they are predicted to be impossible to detect by conventional experimental methods. But Feng and his colleagues point to some alternative tests to prove their existence. They found that observations of old stars and the cosmic microwave background of the universe can reveal clues for superWIMPs.

"One place to look for evidence is in the cosmic microwave background, which in essence is the afterglow of the Big Bang," Feng said. "This background is very uniform. But according to our theory, WIMP decay would set loose a zoo of particles that would create deviations in this background. If such deviations are found, they would provide a particle fingerprint for the existence of superWIMP dark matter."

Feng and his collaborators are currently investigating hints for superWIMPs in present data and are considering further studies that might provide evidence for the existence of superWIMP dark matter.

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