Gamma rays hint at dark matter
Apr 24, 2012 10 comments
Using a new statistical technique to analyse publicly available data from NASA's Fermi Space Telescope, an astrophysicist in Germany says he may have spotted a tell-tale sign of exotic particles annihilating within the Milky Way. If proved to be real, this "gamma-ray line" would, he claims, be a "smoking-gun signature" of dark matter.
There is a wide body of indirect observational evidence that an invisible substance accounts for some 80% of the matter in the universe. Although physicists can measure the effects that this dark matter has on the visible universe, they have very little understanding of what this mysterious stuff actually is. As well as looking for direct evidence of dark matter by detecting it – or even producing it – here on Earth, researchers are also scouring the skies for signs of the particles that dark matter might produce when self-annihilating. An excess of high-energy positrons (anti-electrons) observed by the Italian-led PAMELA spacecraft in 2008, and confirmed by Fermi last year, might be such a signature. However, it is possible that these positrons are produced by processes unrelated to dark matter.
In contrast, say astrophysicists, a gamma-ray line would leave little room for alternative explanations. The dark-matter particles believed to exist in a halo surrounding our galaxy are slow moving because they have been slowed down as the universe has expanded. As a result, the total energy of the photons produced by the collision and annihilation of two such particles is, essentially, twice the rest mass of a dark-matter particle. Conservation of momentum requires that the energy of each photon equals the mass of one dark-matter particle – and would appear as a very narrow peak, or line, in gamma-ray spectra. This is unlike the radiation emitted by all standard astrophysical phenomena, which have much broader energy distributions.
Something at 130 GeV?
In the latest work, Christoph Weniger of the Max Planck Institute for Physics in Munich looked for such lines in about 3.5 years' worth of gamma-ray observations carried out by the Fermi satellite's Large Area Telescope (LAT). To increase his chances of success he only considered data from those regions of the Milky Way that should generate the highest ratios of dark-matter photons to photons from background sources – according to five different models for the distribution of dark matter within the halo. He also restricted the data to within the 20–300 GeV energy band.
In regions close to the centre of the galaxy, Weniger found that the gamma rays collected by Fermi showed evidence for a line, at about 130 GeV, with a statistical significance of 4.6σ. This dropped to 3.3σ after allowing for the fact that he searched for such a line across finite ranges of space and energy. Put another way, there should be only about a 1 in 1000 chance that the line is due to a statistical fluctuation.
Most dark-matter models predict that this line should be very faint because dark matter does not couple directly with electromagnetic radiation. Photons are instead produced by the annihilation of intermediate particle pairs such as electrons and positrons, but such secondary annihilation is generally considered improbable because the extremely high energies of the particles involved means that they would almost certainly fly apart before they have the chance to combine. There are a few models, however, in which such annihilation is enhanced – one such model, for example, allows for the creation of virtual pairs of particles and antiparticles that are unable to fly apart. "If this is a dark-matter signal it would imply a model where the line is surprisingly strong," says Weniger. "This would allow us to reduce the number of possible models considerably."
Weniger acknowledges that his gamma-ray line is provisional, pointing out that it consists of data points from only about 50 photons, and that reaching the roughly 5σ level needed to claim a discovery is likely to need several more years' worth of data. He also points out that because his analysis is based only on publically available data he does not know all there is to know about possible instrumental errors.
Fake gamma-ray lines
In fact, according to Elliott Bloom of Stanford University in the US and Jan Conrad of Stockholm University in Sweden, both members of the Fermi LAT collaboration, instrumental biases associated with identifying photons against a background of charged particles and sifting those photons according to their energy has previously created fake gamma-ray lines, and overcoming these problems, they say, "still requires considerable additional work".
If Weniger's gamma-ray line is real but turns out to be significantly broadened, he believes it could be caused by a more conventional astrophysical process. One potential candidate, he says, which was only identified in 2010 using Fermi data, is a pair of enormous gamma-ray-emitting "bubbles" extending outwards from the plane of the Milky Way. Because the energy of the photons within the bubbles may have a sharp cut-off at around 130 GeV, he suggests that the mechanism responsible for the bubbles might also generate the line that he has identified. Conversely, he says, dark matter might cause the cut-off seen in these bubbles.
John Wefel, an astrophysicist at Louisiana State University in the US, points out another tantalizing possibility: that the peak at about 125 GeV seen in data from the Large Hadron Collider (LHC) at CERN last year, and which is reckoned to be a signature of the Higgs boson, may in fact be related to the line identified by Weniger. "Do we have a halo of Higgs bosons around the galactic centre, or is the LHC observing some new particle likely to be the dark matter particle, and not the Higgs at all?" he asks, playfully.
Weniger's analysis is described in arXiv:1204.2797.
About the author
Edwin Cartlidge is a science writer based in Rome