The Resurs DK-1 spacecraft, which houses the PAMELA experiment, during a test before its June 2006 launch. Credit: Italian National Institute for Nuclear Physics
When dark matter is destroyed, it leaves behind a burst of exotic particles, according to theory. Now scientists have found a possible signature of these remains. The discovery could help prove the existence of dark matter and reveal what it's made of.
No one knows what dark matter is, but scientists think it exists because there is not enough gravity from visible matter to explain how galaxies rotate.
An Italian satellite called PAMELA (Payload for Antimatter Matter Exploration and Light nuclei Astrophysics), launched in 2006 to measure radiation in space, found an overabundance of particles called positrons, which are the antimatter counterpart to electrons (matter and antimatter annihilate each other).
This positron signature could have a variety of causes, but a prime candidate is dark matter, the intangible stuff thought to make up about 98 percent of all matter in the universe. When two dark matter particles collide they can sometimes destroy each other and release a burst of energy that includes positrons.
"PAMELA found a number of positrons much higher than expected," the mission's principal investigator Piergiorgio Picozza told SPACE.com. "Many think this could be a signal from dark matter, because for positrons this behavior fits very well with many theories of dark matter."
The finding, detailed in tomorrow's issue of the journal Nature, is not a total surprise, but it could be a huge splash, if confirmed.
"This kind of signal for dark matter has been predicted as a possible leading signature for over two decades, and [the PAMELA scientists] are seeing just the kind of things one might expect," said University of Michigan astrophysicist Gordon Kane, who was not involved in the research. "There's a very good chance that this is the most important discovery in basic physics for decades."
Positrons are often created when cosmic rays interact with atoms in the gas and dust between stars. But this source cannot produce enough positrons to account for PAMELA's findings. Another possibility is that the positrons PAMELA found were produced by dense spinning stars called pulsars. To distinguish between this option and dark matter, more data will be necessary, either from PAMELA or from the Fermi Gamma-ray Space Telescope, launched last year.
"We hope to have detected dark matter, but now we need other verification coming from other experiments," Picozza said.
Even so, some scientists are excited to have come so close to possibly discovering the presence of dark matter, which has eluded researchers since it was first conceived in the 1930s.
Kane emphasized that the results, though still not certain, could be significant not just as proof that dark matter exists, but also for clues about what makes up this mysterious substance, which cannot be directly seen and is only detected via its gravitational tug on other things.
Kane's personal bet for the particle behind dark matter in these findings is called a wino (pronounced WEE-no) — a specific type of neutralino, which is a theorized category of particles that could exist as "supersymmetric partners" for all the Standard Model particles such as electrons, quarks, etc. The wino is the supersymmetric partner of a particle called the W boson.
"It does particularly well at producing positrons in the annihilation, and the positrons have energies that are about right for these results," Kane said in a phone interview.
If dark matter is made up of neutralinos, then dark matter particles would be their own antimatter particles, because the anti-neutralino is simply a neutralino. Thus, when two dark matter particles collide, they can self-destruct like any other interaction of matter and anti-matter.
Luckily, this does not happen very often. Dark matter particles are thought to be extremely tiny, and the chances of them hitting each other perfectly square on, and under the right conditions for destruction, are very low. This fact allows dark matter to clump together throughout the universe, scaffolding up galaxies and clusters, without destroying itself every time two dark matter particles come near each other.
Even though annihilations are rare, the positrons they produce could survive for up to a few million years, so they can stick around long enough for detectors like PAMELA to find them.