Published online 26 November 2008 | Nature | doi:10.1038/news.2008.1258

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Proton discovery may cast doubt on dark-matter theories.

The Milagro detector has seen cosmic-ray hot-spots. Milagro / U. Maryland / LANL

Hot on the heels of speculation that cosmic rays may have revealed the signature of elusive dark matter in space, new observations could challenge that idea and reinforce an alternative explanation.

A seven-year-long experiment at the Milagro cosmic-ray detector near Los Alamos, New Mexico, has revealed 'bright patches' of high-energy cosmic rays in the sky1 – something incompatible with a dark-matter source.

Cosmic rays are charged particles, mostly protons and electrons, that are produced in space and generally have a characteristic energy spectrum — the higher their energy, the rarer they are.

But last week, researchers working on the Advanced Thin Ionization Calorimeter (ATIC) experiment, which uses detectors borne by a high-altitude balloon to measure cosmic-ray electrons above the Antarctic, reported an unexpected bump in this energy spectrum, corresponding to a surfeit of electrons with energies between 300 and 800 gigaelectronvolts2.

Hints at such an anomaly have been seen before. A satellite observatory — Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) — detected an excess of cosmic-ray positrons, the anti-particles of electrons, at similar energies3. And a Japanese detector called the Balloon-borne Electron Telescope with Scintillating fibers (BETS) also found a small excess of cosmic-ray electrons at high energy4.

These cosmic rays may be the decay products of hypothetical particles of dark matter, thought to make up about 85% of all matter in the Universe. Astronomers have invoked dark matter's gravitational effects to explain why rotating galaxies don't fall apart as they whirl through space. But as the name implies, dark matter can't be seen directly and its identity remains obscure.

A common assumption is that dark matter consists of a hitherto unknown particle that interacts weakly with other forms of matter. In some theories, two dark-matter particles are predicted to annihilate when they collide, producing a high-energy electron–positron pair. These could account for the ATIC cosmic-ray bump and the hints of it in the PAMELA data. But if that's so, the anomalous cosmic rays should be distributed more or less evenly across the sky.

In contrast, the Milagro team, led by Jordan Goodman at the University of Maryland, College Park, found cosmic-ray protons bunched up in two 'hot spots': one between the Orion and Taurus constellations, the other near Gemini. They think that the excess cosmic rays may be coming from exotic sources such as the rapidly rotating neutron stars known as pulsars, rather than dark-matter annihilations.

Dark-matter mystery

Goodman stresses that it's not yet clear if the ATIC and Milagro results are related, because the former measure cosmic-ray electrons whereas the latter detect protons. But he says the sources of the protons they have seen could also plausibly generate the electrons and positrons found in the earlier studies. "If it's the same phenomenon making them all, then it's not dark matter," he says.

But the dark-mater explanation still cannot be ruled out. "I've been totally perplexed by the hot spots but I don't see any reason to connect them with the ATIC findings," says Dan Hooper, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Illinois.

The Milagro detector isn't aimed primarily at investigating cosmic rays, but is instead used for gamma-ray astronomy. When high-energy gamma rays hit our atmosphere they trigger a shower of exotic particles. These particles annihilate when they collide with water in Milagro's giant tank, producing a flash of light that can be recorded by sensors.

But 99.9% of the flashes seen by Milagro originate from collisions of cosmic-ray protons, explains Goodman. That creates a background signal that has to be subtracted in order to identify gamma rays from energetic astrophysical sources. Goodman says that finding localized sources of cosmic-ray protons in this background came as a surprise to them.

An earlier cosmic-ray experiment called the Tibet Air Shower Array, run by a team of researchers in Japan and China, saw broad differences in the cosmic-ray intensity between the two hemispheres5, but no one had previously seen such smaller-scale concentrations.

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The Milagro team suggests that the protons, with energies of around 10,000 gigaelectronvolts, may be generated in the extreme astrophysical environment of a super-dense neutron star or pulsar. At least some high-energy cosmic rays have previously been shown to come from super-massive black holes in nearby galaxies6.

"We don't know what is causing it," Goodman admits. He suggests that the localization may be partly caused by magnetic fields focusing the protons' trajectories.

But in general, magnetic fields in interstellar space should exert a randomizing influence, destroying any bright spots, says Hooper. "I can't imagine how they're created, and I don't know if anyone has any great ideas," he says.