Published online 26 February 2010 | Nature | doi:10.1038/news.2010.97


A CoGeNT result in the hunt for dark matter

An underground experiment may have detected a type of dark-matter particle.

The MINOS detector in the Soudan Underground Mine State Park, during (re)construction.The Soudan mine hosts both the CDMSII and CoGeNT dark matter experiments.J. Davis/Wikimedia

Deep in the Soudan mine in Minnesota, some 700 metres below ground amid the bones of bats, sits the huge Cryogenic Dark Matter Search (CDMSII) experiment, which at its heart contains a rack of supercooled hockey-puck-sized silicon and germanium detectors nestled within Russian-doll layers of shielding.

Two weeks ago, the CDMSII collaboration published a paper showing that two particles had penetrated its detector's defences — particles that, given the lack of any other particle activity down in the frigid quiet of the detectors, looked very much like dark matter1. Dark matter is thought to make up 85% of the mass in the Universe, but has not been detected directly — quite. The attention-grabbing claim of the CDMSII collaboration has many physicists thinking — but not yet convinced — that the team could be on to something.

Just a stone's throw from the CDMSII experiment, across the subterranean cavern, lies a far smaller box that is thickening the dark-matter plot. The box contains a single germanium hockey puck, similar to those in the CDMSII experiment but operated by the Coherent Germanium Neutrino Technology (CoGeNT) collaboration and tuned to detect incoming particles with much lower masses than the CDMSII. It began collecting data in December 2009, and, after just 56 days, the group is reporting hundreds of particle strikes that cannot be explained other than by invoking dark matter.

"If it's real, we're looking at a very beautiful dark-matter signal," says Juan Collar, a physicist at the University of Chicago and CoGeNT spokesperson. Collar presented the work today at a dark-matter conference at the University of California, Los Angeles. The results were posted on the preprint server Arxiv yesterday2.

“If it's real, we're looking at a very beautiful dark-matter signal.”

Juan Collar
University of Chicago

Confirmation of the result — and Collar is careful to say that it is still early days — would radically shift attention to experiments that are sensitive to lower energies. The CoGeNT experiment looks for a type of dark-matter particle called a WIMP, or Weakly Interacting Massive Particle. The new data point to a WIMP with a mass in the range of 7–11 billion electronvolts. Theorists have conjured up hundreds of mathematically consistent models for producing WIMPs of different masses in the early Universe, and the particles detected by CoGeNT fit well in the realm of the theoretically possible.

But the majority of models had favoured WIMP particles that are an order of magnitude heavier, and some experiments — such as the CDMSII, and those using large tanks filled with mineral oil or liquefied noble gases — were aiming for that territory. "The experiments designed to look at the heavier particles aren't going to like the CoGeNT result," says Dan Hooper, a theorist at the Fermi National Accelerator Laboratory in Batavia, Illinois.

Swimming in dark matter

On its own, the CoGeNT result is just yet another tentative claim in a sea of dark-matter hints. And Collar himself acknowledges that the CoGeNT detector is not shielded as well as the CDMSII experiment, so the signal he's seeing could simply be an unexplained radioactive decay process in the electronics. But the fact that the CoGeNT results mesh so well with those of the CDMSII and one other experiment is what gets the attention of Neal Weiner, a theorist at New York University. "I think this will make some noise," says Weiner. "It lines up nicely with the possible interpretation of other results."

The WIMP called for by the CoGeNT result is consistent with the two CDMSII 'hits', but because those events lay at the lower reaches of CDMSII's energy sensitivities, researchers will have a hard time sifting for more.


The second experiment that matches up with the CoGeNT result is the strange signal seen in a deep underground detector in Gran Sasso, Italy, by the DAMA/LIBRA collaboration (Dark Matter Large Sodium Iodide Bulk for Rare Processes) (see 'Italian group claims to see dark matter - again'). For a decade, the Italian team has found a periodic signal that won't go away; the effect has been put down to Earth annually passing through a steady current of dark matter in the Milky Way.

The particles seen by CoGeNT are much lower in mass than the regions of parameter space currently being investigated by space satellites such as the Fermi Gamma-ray Space Telescope and the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), which have also found dark matter hints — but for heavier particles (see 'Dark matter intrigue deepens').

But Weiner points out that whereas the CoGeNT result will raise doubts about what the space experiments are seeing, they won't undermine them completely; it's entirely possible that all of the dark matter seen indirectly in the cosmos could be comprised of both light and heavy particles. So he doesn't want the planned higher-mass detectors to be mothballed just yet. "This won't put any of the detectors out of business," says Weiner. "We should be cautious." 

  • References

    1. The CDMS II Collaboration Science advance online publication doi:10.1126/science.1186112 (2010).
    2. Aalseth, C. E. et al. Preprint at (2010).
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