Published online 19 November 2008 | Nature 456, 290-291 (2008) | doi:10.1038/456290b

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Finding from balloon experiment adds to satellite data.

A high-altitude balloon experiment above the Antarctic seems to have seen a possible signature of mysterious 'dark matter', similar to that spotted earlier this year by a European satellite.

The Advanced Thin Ionization Calorimeter (ATIC), an experiment to search for charged particles from space, has spotted a surplus of high-energy electrons coming from somewhere in the cosmos (see Letter, page 362, and News & Views, page 329). Although the interpretation is far from certain, the electrons could be produced by dark matter — previously undetected particles that physicists believe make up 85% of all matter in the Universe.

ATIC's findings are similar to data from the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) satellite mission, a collaboration between Italy, Russia, Germany and Sweden that spotted an excess of high-energy positrons, or anti-electrons, at roughly similar energies (see Nature 454, 808; 2008). "In several respects the two measurements complement each other," says John Wefel, head of the ATIC collaboration and a physicist at Louisiana State University in Baton Rouge.

Several experiments may have found dark matter, the existence of which is inferred here (blue). NASA

Wefel's team looked at data from two multi-day ATIC missions flown between 2000 and 2003 some 35 kilometres above the Antarctic ice. With 99.5% of Earth's atmosphere beneath them, the missions measured electrons that come from various galactic sources such as exploding stars. As predicted by theory, the experiment saw fewer electrons at higher energies. But between 300 gigaelectronvolts and 600 gigaelectronvolts, the number of electrons rose sharply before falling off to background levels.

PAMELA's results, as posted on the arXiv preprint server last month (O. Adriani et al. http://arxiv.org/abs/0810.4995 ; 2008) and also submitted to Nature, show positrons increasing up to 100 gigaelectronvolts. Although the collaboration did not report beyond this energy level, some suspect that the number of positrons may continue to increase at higher energies. And, because the PAMELA and ATIC data were measured in different ways, researchers believe that their findings probably confirm each other.

The 'bump' ATIC sees in the number of electrons could be the result of heavy dark-matter particles annihilating, according to Wefel. When this happens, two dark-matter particles collide and their mass is converted to pairs of fast-moving electrons and positrons, the energies of which would correspond to the mass of the original particles.

“Annihilation of dark-matter particles is certainly the sexiest of the possibilities.”



"That's certainly the sexiest of the possibilities," says Dan Hooper, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Illinois. The exact nature of the dark-matter particles that produce electrons is uncertain, but one idea is that they may be ordinary particles that spend part of their lives in a compact extra dimension of space. Whereas the particles would appear relatively stationary to observers trapped in three spatial dimensions, they could be moving at ultra-high speeds in a fourth spatial dimension. At high speeds, they would create a gravitational force that could be felt by matter trapped in three dimensions of space-time. "It's very wild," Hooper says.

Other, more mundane reasons might also explain the data. The leading candidate is a nearby pulsar, says Aldo Morselli, a particle physicist from the Italian National Institute of Nuclear Physics at the University of Rome, Tor Vergata. Pulsars, the fast-spinning remnants of supernovae explosions, have enormous magnetic fields that can accelerate electrons to the high energies seen in the experiments, although ATIC's bump is harder to match to a pulsar than PAMELA's increase.

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Wefel agrees that it is too early to say for certain whether ATIC and PAMELA have seen dark matter. "You cannot make a hard and fast claim," he says. "The case is still open."

But more evidence may come soon. NASA's Fermi telescope (formerly known as GLAST), which launched in June this year, is designed to hunt for high-energy γ-rays. But the telescope can also spot electrons and positrons, according to Morselli. In the coming months, he predicts that the telescope will be able to verify the positron and electron data of PAMELA and ATIC. In addition, Fermi may be able to spot γ-rays that have come from dark-matter annihilations. "By the spring of 2009, we'll have a lot more information," Morselli says.