Mysterious electrons may be sign of dark matter

Dark matter is proving less shadowy than its name suggests. Its signature may have been detected by a balloon-borne experiment that measured a surprisingly high number of energetic electrons streaming in from space.

High-energy electrons are found throughout space and are accelerated when stars explode in supernovae. But a balloon-borne detector flying over Antarctica called the Advanced Thin Ionization Calorimeter (ATIC) has detected 70 more high-energy electrons than the normal background level attributed to supernova blasts.

John Wefel of Louisiana State University in Baton Rouge, who led the collaboration, says there are two possible explanations.

The electrons could come from a nearby astrophysical object, such as a pulsar, that lies within 3000 light years from Earth. But the team has spent four years trying to fit the signal to such an object and has yet to find a good match.

The alternative is that the electrons were produced when two dark matter particles met and destroyed each other. That hypothesis is strengthened by the electrons' observed energies, which range from 300 to 800 gigaelectronvolts.

"There is nothing that we know of in high-energy physics or astrophysics that happens in this energy range," says Wefel.

What's more, the signal peaked at 650 GeV and then rapidly declined to the background level at 800 GeV. According to Wefel, this is the kind of signature you would expect if a type of exotic particle known as a Kaluza Klein particle was the dark matter culprit, with the peak at 650 GeV corresponding to its mass.

This type of particle is a WIMP (weakly interacting massive particle), one of the most promising candidates for dark matter, and comes from theories in which the universe has extra spatial dimensions. These extra dimensions can only be detected by observing WIMPS that have leaked into the four dimensions (three of space and one of time) that are familiar to us.

The past few years have been good for dark matter hunters. In 2007, NASA's WMAP satellite, which measures the big bang's afterglow, picked up an excess of microwaves from around the centre of our galaxy. This 'WMAP haze' could be radiation produced when dark matter particles collide. ... - ns