Published online 5 May 2009 | Nature | doi:10.1038/news.2009.444

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Space telescope may have glimpsed hint of mystery particles.

New data from two experiments — one in space, one on a balloon floating above Antarctica — hint at a tantalizing detection of dark matter, the mysterious stuff comprising 85% of the universe's matter. The evidence is a reported excess of high-energy electrons and their antimatter counterparts, positrons, which could be created as dark matter particles annihilate or decay.

The signal from Fermi, the orbiting gamma-ray telescope, is subtle, whereas that claimed by the balloon-borne Advanced Thin Ionization Calorimeter (ATIC) is much more pronounced. The differences are puzzling, but the findings — according to some — could herald the birth of a new age of dark matter exploration.

Fermi's true insight into dark matter may come from its gamma ray studies. NASA

"We may very well be seeing the beginning of the discovery era," says Dan Hooper, a theorist at Fermi National Accelerator Laboratory in Batavia, Illinois, who is not affiliated with either of the experiments.

Peter Michelson, principal investigator for the instrument on Fermi that made the detection, cautions that his group is not yet claiming to have found a smoking gun for dark matter. The signal could also come from more mundane sources nearby, such as pulsars, the spinning remnants of supernovae. "But if it isn't pulsars, it is some new physics," says Michelson, of Stanford University in California.

The Fermi results were presented at a meeting of the American Physical Society in Denver, Colorado, on 2 May, and published this week in Physical Review Letters.1

The ATIC results, published in Nature in November2, are more provocative. They show a specific spike in the energy of excess electrons between 300 and 800 gigaelectronvolts (GeV), a peak that could point to the mass of any associated dark matter particle. At first, the Fermi and ATIC results would seem incompatible, because Fermi doesn't see that same sharp increase in the 300 to 800 GeV range. With the Fermi data, "this [ATIC result] now seems unreasonable," says Mark Pearce of the Swedish Royal Institute of Technology and a team member on a related experiment, PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics).

But ATIC isn't backing down from its claims. The work published in November was based on two balloon flights. On 4 May, T. Gregory Guzik, of Louisiana State University in Baton Rouge, presented preliminary results from a third flight in December 2007, made with a better detector. The sharp peak was still there. He is not, in any event, sure that the two results really are incompatible; ATIC has more certainty in knowing the energy of its electrons in its sharp peak, while Fermi has trapped far more of them in building its subtle bump. "My suspicion is that we might come down and Fermi might go up," says Guzik.

The third experiment, PAMELA, seems more consistent with the Fermi results so far. It has spotted a rise in the fraction of positrons, which being more rare could be a clearer signal for dark matter.3 But PAMELA has only reported data up to energies of 100 GeV, not as high as Fermi or ATIC.

The effect of gamma rays

All of the experiments so far have probed dark matter only indirectly. And the signal can be blurry and smeared, since electrons whizzing through space lose energy as they are whipped around by galactic magnetic fields.

One way around that problem would be to study gamma rays, which are also produced in many dark matter annihiliation and decay scenarios but are immune to magnetic fields. So many think that Fermi scientists, who made their first dark matter claims based on electron data, will actually do the definitive work with its gamma-ray detectors, which could not only determine the mass of dark matter particles more precisely, but also determine where the signal is coming from in space.

The strongest gamma-ray signal is expected at the centre of the Milky Way, but that is also where separating the signal from confounding sources will be the most difficult. Fermi is also looking for clumps of dark matter hovering near the plane of the Milky Way – where researchers on 3 May reported tantalizing evidence of a 'hotspot' of gamma rays, which is likely to disappear with further scrutiny but hints at the gamma-ray findings to come. And finally, Fermi may also turn its eyes to the galactic poles and stare off into extragalactic space, where it may find evidence for a faint web of dark matter. This could be the most important in constraining the cosmological models that predict the growth of structure in the universe — how galaxies coalesced around initial dark matter seeds.

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But all of that will require at least a year's harvest of Fermi data. And for now, the electron data seems to be nourishing the community just fine. By Monday, theorists had already readjusted their dark-matter models to be consistent with the new results from Fermi. For many years, models commonly predicted that the energy of dark matter particles centered around 100 GeV. But the new data — especially Fermi's — are favouring models that would produce particles at least an order of magnitude heavier, on the scale of teraelectronvolts.

That not only means that Fermi, with its ability to probe in that energy range, will be crucial in nailing down dark matter, but it also has implications for particle colliders such as the Large Hadron Collider in Geneva, Switzerland. Many at the LHC were hoping to generate a few particles in their collisions. But the higher the mass, the harder it will be for the LHC to see them.

If the current Fermi results stand up, says Hooper, "whatever is behind dark matter will be very hard for the LHC to see."