Published online 17 December 2009 | Nature | doi:10.1038/news.2009.1151

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Two direct hits in dark matter hunt

Subterranean detector may have nabbed elusive particles.

CDMSHas the Cryogenic Dark Matter Search finally found its elusive quarry?R. Hahn/Fermilab

An experiment buried deep beneath the forests of Minnesota has seen two events that could be the first direct detection of dark matter.

The finding is tentative — it's still possible that conventional particles might be causing the signal — but, if confirmed, it will mark the end of a decades-long search for the mysterious particles. The data from the Cryogenic Dark Matter Search II (CDMSII) experiment also suggests that dark matter should show up in collisions at the Large Hadron Collider (LHC) — the world's most powerful particle accelerator, based at CERN, Europe's particle-physics laboratory near Geneva, Switzerland.

Observations by Swiss astronomer Fritz Zwicky in the 1930s gave the first hints of the existence of dark matter. Since then, studies of the Universe's structure, and of the way galaxies rotate, have confirmed that there must be an unseen form of matter shaping the cosmos. This dark matter is believed to make up as much as 85% of all the material in the Universe, although its identity is unknown.

The two events, seen by CDMSII in 2007, are the signatures of the most likely form of dark matter, known as weakly interacting massive particles (WIMPs). Each particle could be as massive as an entire atom, yet reveal itself only rarely by interacting with conventional matter.

The possible WIMPs were caught in the experiment's crystals of germanium and silicon, which are chilled to nearly absolute zero (–273.15 ºC). When a WIMP strikes one of the crystals, it should trigger vibrations that raise the temperature of the detector very slightly; it should also create a small charge on the crystal's surface. Comparing the size and timing of the two signals can help to determine whether or not they were caused by WIMPs.

"An interesting hint"

CDMSII scientists are remaining tight-lipped about the results until they are peer-reviewed. But in a series of talks delivered over the next few days in the US and Europe, they are expected to announce that their WIMP candidates have a mass of 30–60 gigaelectronvolts — roughly 30–60 times that of a single proton. From their analysis, they believe that there is a 75% chance that both events are WIMPs, and a 25% chance that they might both be false-positives caused by stray radiation.

A three out of four chance is not good enough to claim a definitive detection of the elusive WIMPs, says Timothy Sumner, a physicist at Imperial College London, who leads a rival WIMP experiment known as Zeplin-III. "Statistically, it's not compelling," he says. The big question will be whether the experiment has adequately accounted for background radiation.

Despite being shrouded by lead bricks and located three-quarters of a kilometre underground in the disused Soudan Mine, there's still a chance that a few neutrons might find their way into the crystals and mimic WIMP signals. Stray electrons or gamma rays striking the detector's surface could also create false positives. Understanding and accounting for these backgrounds is extremely difficult and is crucial for confirming any detection, Sumner says.

Seeing two events is a tantalizing and frustrating outcome for CDMSII. Had the detector detected five WIMPs, physicists could have confidently claimed a discovery, while seeing no events at all would have further narrowed down the possible mass of the particles. "The best we could call it is a hint," says John Ellis, a theoretical physicist at CERN. "An interesting hint."

The hunt is on

The possible detection at CDMSII is the latest in a series of possible dark-matter sightings. In August 2008, an Italian-led satellite-based experiment known as PAMELA saw an excess of antielectrons (positrons) that could stem from the annihilation of dark-matter particles (see 'Physicists await dark-matter confirmation'). And in October, a NASA satellite known as the Fermi Gamma-ray Space Telescope saw a haze of high-energy light in the centre of our Galaxy that could also be a dark-matter signature (see 'Bright light hints at a dark centre to the Galaxy'). An Italian experiment known as DAMA has claimed to see dark matter before, but many physicists are sceptical of the group's findings (see 'Dark-matter test faces obstacles'). The new results are consistent with the observations from PAMELA and Fermi but make the DAMA results less likely to be valid.

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The CDMSII result will now spur physicists at the LHC to verify the sighting. "The LHC would see this very easily and relatively quickly," Ellis says, adding that the collider could potentially generate a detectable WIMP signal by the end of next year. Confirmation of WIMPs at the energies suggested by CDMSII would also provide support to supersymmetry, a popular theory that would simultaneously explain dark matter and unify several fundamental forces.

But scientists may not have to wait even that long to learn whether CDMSII is right. Experiments such as Zeplin-III are now taking data and might shore up the CDMSII observation within a matter of months. "A possible confirmation could happen very quickly," Sumner says. 

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