Published online 9 July 2008 | Nature 454, 148-149 (2008) | doi:10.1038/454148b

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Super-sensitive tool key to dark-matter claim

Theorists unlock mysteries of experimental results.

It's one of the most controversial experiments in physics, but an Italian group's claim to have seen dark matter may be vindicated after all. A spate of theoretical papers can explain why the Italians see a signal where others do not.

Dark matter interacts rarely — if at all — with everyday molecules and atoms. Although it is thought to make up some 82% of the matter in the cosmos, scientists have so far only seen dark matter indirectly, through its gravitational pull on more conventional objects, such as galaxies.

Groups around the world are racing to spot dark matter directly, but only one claims to have actually seen it. In April, the DAMA/LIBRA (Dark Matter/Large Sodium Iodide Bulk for Rare Processes) experiment, located deep beneath Italy's Gran Sasso mountain, announced that it had evidence of dark-matter particles. The claim, the group's second in less than a decade, was criticized for being incompatible with rival work (see Nature 452, 918; 2008).

"It looked at the time like they were completely inconsistent," says Kathryn Zurek, a theorist at the University of Wisconsin at Madison. But over the past three months, Zurek and other theorists have begun to find reasons why DAMA might be seeing dark matter that their rivals cannot. The papers1, 2, 3 have been trickling onto the popular arXiv preprint server.

At the core of most of the papers is a previously unknown effect that DAMA claims to have seen. Known as 'channelling', it means that DAMA is much more sensitive to lightweight particles than previously thought. If dark matter were made of these lighter particles, then DAMA would see it but its rivals would not.

Just what those particles could be is at the centre of the papers now peppering the arXiv. For her part, Zurek thinks that they could be unusually light neutralinos1, partners of neutrinos predicted by a popular theory known as super-symmetry.

Other papers wander farther afield to explain the discrepancy. Robert Foot, a theorist at the University of Melbourne in Australia, says that the particles would be consistent with his theory of mirror matter2, in which a hidden world of mirror atoms and molecules exist alongside our own. The hypothesis is part of a broader set of theories known as 'hidden-sector' models, which postulate a range of exotic hidden particles that interact with each other but not with the visible Universe.

Hints of hidden sectors would also show up in future γ-ray experiments and at the Large Hadron Collider at CERN, the European particle-physics laboratory, says Jonathan Feng, at the University of California in Irvine. "If this is the right explanation, it's going to be strongly verified in the next 6–12 months," he says.

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"Theoretical and phenomenological papers are always very useful," says Rita Bernabei, a physicist at the National Institute of Nuclear Physics in Rome, who heads the DAMA experiment and stands by its findings.

But other experimentalists remain sceptical. Smaller experiments using sodium iodide have failed to see the channelling effect, says Timothy Sumner, a physicist at Imperial College London. And the theoretical solutions that require lighter-weight particles do not fit with the favourite versions of super-symmetry theory. "I suspect it will be necessary to get additional data to bring the community around," he says. 

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