Doubt cast on Fermi's dark matter smoking gun

It was hailed as a smoking gun for dark matter, raising hopes that we might finally pinpoint the particle that is thought to make up 80 per cent of the mass in the universe. But purported evidence of dark matter interactions in the centre of our galaxy may not be as solid as hoped.

Most physicists think dark matter is made of weakly interacting massive particles, or WIMPs, which only interact with normal matter via gravity. When two WIMPs meet, they should annihilate and spew out new particles, including high-energy gamma rays.

The Fermi Gamma-ray Space Telescope searches for dark matter by seeking these gamma rays. If it detects more gamma rays of a certain energy than known sources can explain, that would be thought by many to be a sign of WIMPs.

"This kind of narrow, distinct feature is not really predicted to come from any other process," says Fermi team member Andrea Albert of Ohio State University.

That's why scientists were abuzz in April, when Christoph Weniger of the Max Planck Institute for Physics in Munich, Germany, reported observing a spike in gamma rays at 130 gigaelectronvolts (GeV). These were coming from the Milky Way's centre and had no obvious astrophysical source.

While not a member of the Fermi team, Weniger was able to analyse publicly available data from 3.5 years of telescope observations. Other physicists looked at the same data and agreed that the signal was strong enough not to be a random fluctuation. Either the telescope was behaving oddly, or it was seeing dark matter particles with energies of 130 GeV.

"If the latter is true, it would dwarf the Higgs boson discovery," wrote physics blogger Jester on his blog Resonaances.

The Fermi team remained quiet about the finding at the time, but their reaction was hotly anticipated. In addition to having a full four years' worth of data to analyse, the team also has access to an important cross-check on any possible signal.

Dark matter should pool at the galaxy's centre, with negligible amounts surrounding Earth. What Earth does have, however, is a ring of gamma rays produced when cosmic rays strike the atmosphere. If, when Fermi is pointed at this ring, Weniger's 130 GeV gamma ray signal shows up there, it suggests that it isn't a dark matter signal. "It's a good place for a sanity check," says Albert.

The team first had to reprocess their data from the galactic centre to account for a glitch caused by a damaged instrument on the telescope. That revealed that the signal had shifted from 130 to 135 GeV, Albert told the Fourth International Fermi Symposium in Monterey, California, on 2 November. What's more, that signal had faded to statistical insignificance.

"The feature's gotten a little smaller," she says. "It hasn't gone away completely, but we do not see it to be very significant. At this point, we have to cast doubt on this being a dark matter line."

The signal also showed up in the ring of gamma rays around Earth, but it seems to account for only half of that detected from the galactic centre, and there is no good way to explain why it is there. "There is nothing obvious, at least to me, that connects the galactic centre and the [gamma-ray ring]," Weniger says.

It could be that there is a fault with the Fermi team's data processing methods, or that one line is false and the other is real. For now, though, Weniger thinks there is still room for the line to be a dark matter sign. "It's difficult to get attached to a feature, because they come and go often," he says. "But it could be the real thing. It could be the most exciting discovery of the decade."

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