A. Mellinger, CMU; T. Linden, Univ. of Chicago/NASA Goddard
It is one of the most disputed observations in physics. But an explanation may be in sight for a mysterious excess of high-energy photons at the centre of the Milky Way. The latest analysis1 suggests that the signal could come from a dark-matter particle that has just the right mass to show up at the world’s largest particle accelerator.
The Large Hadron Collider (LHC), housed at the CERN particle-physics laboratory near Geneva, Switzerland, is due to restart colliding protons this summer after a two-year hiatus (see 'LHC 2.0: A new view of the Universe'). Physicists there have told Nature that they now plan to make the search for such a particle a top target for the collider’s second run.
A positive detection would resolve the source of the galactic γ-rays. But it would also reveal the nature of dark matter, the invisible stuff thought to make up around 85% of the Universe’s matter, and would be long-sought evidence for supersymmetry, a grand way to extend the current standard model of particle physics.
“This could very well be the single most promising explanation for the Galactic Centre proposed to date,” says Dan Hooper of the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, although he adds that “there are quite a few others that are not too far behind”.
In 2009, Hooper and Lisa Goodenough, then a graduate student at New York University, were the first to spot the signal2, in data from NASA’s Fermi Gamma-Ray Space Telescope. They proposed that the bump was a signature of dark matter. Two colliding dark-matter particles would annihilate each other, just as ordinary matter does with antimatter. The annihilation would generate a succession of short-lived particles that would eventually produce γ-rays.
But the proposed particle, which has been dubbed the hooperon or gooperon after its proponents, soon ran into problems with physicists’ favourite version of supersymmetry. Although the minimal supersymmetric standard model (MSSM) allows for dark-matter particles with the estimated mass of hooperons — about 25–30 gigaelectronvolts (1 GeV is roughly the mass of a proton) — multiple experiments had suggested that the particles must be heavier. To accommodate hooperons, MSSM would have to be modified to an extent that makes many physicists uncomfortable. “It would have required a completely new theory,” says Sascha Caron, a particle physicist at Radboud University Nijmegen in the Netherlands, who leads the team behind the latest calculations.
Sceptics suggested that the γ-ray excess spotted in the Fermi data had more-mundane explanations, such as emissions from neutron stars or from the remnants of exploded stars.
But in late 2014, it emerged that calculations for the range of dark-matter-particle masses that would be compatible with the Fermi bump were too conservative. Fresh estimates of the γ-ray ‘noise’ produced by known sources, provided by the Fermi science team3 and others4, allow for much heavier particles. “The excess can be explained with a particle of up to 200 GeV,” says Simona Murgia, a physicist at the University of California, Irvine, and a leading scientist in the Fermi team.
Armed with this insight, Caron and his collaborators recalculated the predictions of the MSSM theory and found another potential explanation for the excess — an existing dark-matter candidate called a neutralino. The neutralino was heavy enough not to be excluded by previous experiments, yet light enough to potentially be produced in the second run of the LHC.
“This could very well be the single most promising explanation for the Galactic Centre.”
Caron’s model also produces a prediction for the amount of dark matter that should have been created in the Big Bang that is compatible with state-of-the-art observations of the cosmic microwave background — the relic radiation of the Big Bang — performed by the European Space Agency’s Planck probe (see Nature http://doi.org/38k; 2014). This cannot be a coincidence, he says. “I find this quite amazing.”
Caron’s team is not the only one reanalysing the Fermi bump in light of the new estimates. Similar but less-detailed calculations done by Fermilab physicist Patrick Fox and his colleagues last November5 also revealed the neutralino as a potential cause of the Fermi γ-rays. And Katherine Freese, director of Nordita, the Nordic Institute for Theoretical Physics in Stockholm, says that she and her collaborators have calculated that the excess could be caused by a type of dark matter that features in a less-popular theory of supersymmetry.
Resolution may be just around the corner. In addition to being produced at the LHC, the neutralino could also be within the shooting range of next-generation underground experiments that are trying to catch dark-matter particles that happen to fly through Earth, says physicist Albert De Roeck, who works on the CMS, one of the two LHC detectors that will hunt for dark matter. If such a particle is indeed the cause of the γ-rays, he says, “it seems that the dark-matter signals should be observed very soon now”.
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