Published online 13 August 2008 | Nature 454, 808-809 (2008) | doi:10.1038/454808b

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Physicists await dark-matter confirmation

PAMELA mission offers tantalizing hint of success.

Rumours are swirling that a European satellite mission may have detected dark matter, the mysterious particles thought to make up as much of 85% of all matter in the Universe.

Nature has learned that the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) mission — a collaboration between Italy, Russia, Germany and Sweden — has detected a surplus of high-energy antielectrons whizzing through space. The antielectrons, also called positrons, could be the clearest signature yet of the dark matter lurking in the Milky Way, according to Dan Hooper, a theoretical physicist at Fermilab in Batavia, Illinois. “If it's true, it's a major discovery,” he says.

Previous space and balloon missions saw hints of the same positron surplus in the 1990s. But their energy range was limited and their measurements had high uncertainty. PAMELA is, in principle, capable of detecting higher-energy positrons with far better accuracy than any other mission to date.

PAMELA's principal investigator, Piergiorgio Picozza, a physicist at the University of Rome Tor Vergata, declined to comment on the findings until they have been published. However, those who have seen the PAMELA data think they are telling. “This is the type of signal that one would expect [from dark matter],” says Graciela Gelmini, a theoretical physicist at the University of California, Los Angeles, who has studied the team's results. But Gelmini adds, the measurement is tricky, “so one has to be cautious”.

Most physicists believe that the Universe is filled with particles that rarely — if ever — interact with the stuff of stars, planets and people. This 'dark matter' has so far only been indirectly detected through its gravitational pull on galaxies, and its influence on the curvature of the Universe itself.

But PAMELA may be seeing another, more concrete signature of the ghostly particles. The satellite has found an unexpected increase in the number of high-energy positrons flitting through the Galaxy, says Bob McElrath, a theorist at CERN, Europe's high-energy physics laboratory near Geneva in Switzerland. The findings were briefly flashed across the screen at this month's International Conference on High Energy Physics in Philadelphia, Pennsylvania, and they've got the community abuzz, McElrath says. “I've had people e-mailing and asking me to try and get the slides.”

PAMELA's positron surplus could come from dark matter particles described by the supersymmetry theory. This predicts the existence of super-heavy counterparts to everyday particles. The lightest supersymmetric particles are expected to be both massive and stable — making them prime candidates for dark matter. Occasionally, theorists believe, two of these particles will smash together and annihilate each other in a burst of energy. The annihilation will create a stream of more conventional particles that will eventually decay, leaving energetic electrons and positrons. In other words, the positrons detected by PAMELA could be the direct result of dark-matter annihilations.

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The detection is the second dark-matter claim by Italians in recent months. In April, scientists at the DAMA/LIBRA (Dark Matter/Large Sodium Iodide Bulk for Rare Processes) experiment, located beneath Italy's Gran Sasso mountain, claimed to have seen dark-matter particles (see Nature 452, 918; 2008). The PAMELA results are unconnected, and their suggested mass range for dark matter seems to contradict the DAMA claim.

PAMELA's findings are far from certain. Identifying fast-moving positrons is extremely difficult, says Stefan Schael, a physicist at RWTH Aachen University in Germany. Their positive charge and high energies can make them look like ordinary protons, he says. Ideally, experiments would have two detectors capable of telling the difference, but PAMELA only has one. “They have only one chance for this identification,” Schael says. “This is the main challenge for the group.”

Even if the surplus exists, it could be from nearby astronomical sources. Objects such as neutron stars, pulsars and X-ray binary stars are capable of making energetic positrons that would fool PAMELA. “When the data are released there will certainly be a large number of people looking at the question,” says Hooper.

For now, however, the community is waiting with bated breath. As McElrath says: “We all wonder what's going on up there.” 

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