Dark matter no-show hobbles elegant particle theory

Lisa Grossman, reporter

(Image: Francesco Arneodo LNGS-INFN)

Dark matter stubbornly refuses to come out of the shadows. The latest results from an underground detector show no sign of WIMPs, or weakly interacting massive particles, the still-theoretical particles thought to make up the invisible majority of the universe's mass.

The result puts the strictest limits yet on the particle's properties, and may squeeze out some favourite extensions to the standard model of particle physics.

Dark matter is the mysterious stuff dreamed up by cosmologists to explain why galaxies don't fly apart and why matter in the universe clusters the way it does. To explain the universe we see, dark matter must make up 83 per cent of all matter. But it doesn't emit light or speak to ordinary matter at all, except through gravity.

Many physicists believe dark matter is made up of WIMPs, particles that carry mass but, as their name suggests, rarely interact with other particles.

Physicists try to glimpse these particles by placing detectors deep underground, where they are shielded from cosmic radiation. If a WIMP collides with the nucleus of a heavy atom such as xenon, the collision should produce a telltale light signal, which the physicists can use to reconstruct the properties of the particle responsible.

The new result comes from the Xenon100 experiment (see photo), a tub of cryogenically cooled liquid xenon buried 1400 metres down a mine at the Gran Sasso National Laboratory near L'Aquila, Italy. The project's previous results, presented over a year ago and based on just 100 days of data, came up empty.

The latest results, presented at the DarkAttack2012 conference in Ascona, Switzerland, on 18 July, were based on 225 days of new data. The team used an improved detector that was 3.5 times as sensitive to the skittish particles as before. Still, the result was the same: no WIMPs.

"We have essentially set the most stringent limits of any other experiment" for WIMPs with masses heavier than 8 gigaelectronvolts (GeV), says Antonio Melgarejo of the Xenon team.

That doesn't mean WIMPs aren't real - it just means they may be harder to find than we hoped. Either WIMPs are much lighter than theories originally predicted (Xenon100 was optimised for 100-GeV WIMPs), or they're even more shy of ordinary matter than thought.

This in turn tightens the noose around certain versions of the theory known as supersymmetry, which some hope will extend the standard model, removing its many insufficiencies. The standard model was completed by the discovery of the Higgs boson this month, but it still doesn't explain dark matter. In supersymmetry, every standard particle has a heavier "superpartner" particle, one of which could be a WIMP.

However, the new Xenon results rule out most of the region where some of the simpler supersymmetry models predicted a dark matter WIMP should be.

"We are starting to explore the region of supersymmetry," Melgarejo says. "It definitely imposes a big problem."

The Xenon team will keep looking. The next generation of the experiment, called XENON1T, will use a tonne of liquid xenon, and should start construction this year.

Also this year, a detector called LUX (Large Underground Xenon) will start operation at a mine in South Dakota. It promises to be 10 times as sensitive as previous searches.

If WIMPs don't show up soon, though, it may be a sign that physicists are simply on the wrong track.

Source: http://feeds.newscientist.com/c/749/f/10897/s/2183a362/l/0L0Snewscientist0N0Cblogs0Cshortsharpscience0C20A120C0A70Cdark0Ematter0Eno0Eshow0Ehobbles0Eel0Bhtml0DDCMP0FOTC0Erss0Gnsref0Fonline0Enews/story01.htm

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