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Thursday, April 17, 2008

Dark matter mystery 'solved' - again

It's the experiment dark matter hunters love to hate. But physicists working on the DAMA (DArk MAtter) experiment might have the last laugh later this week. Rumour has it that, later this week, an Italian lab will claim to have found dark matter, at a conference in Venice, Italy.

News of the announcement has spread like wildfire among physicists here at the American Physical Society conference in St Louis, Missouri. What makes the results so controversial is that DAMA scientists have claimed to detect dark matter before, even though other experiments have failed to see it.

Now the experiment has been revamped to make it more sensitive to dark matter and renamed LIBRA. On Wednesday, Rita Bernabei of the University of Rome Tor Vergata, who leads both the DAMA and LIBRA teams, will present the first findings at the NO-VE workshop on neutrino oscillations.

The controversy dates back to 1998 when the DAMA collaboration claimed it had found evidence for particles of dark matter passing through its detector located in an underground laboratory deep within the Gran Sasso mountain, Italy. Dark matter is the invisible, mysterious stuff thought to make up nearly 90% of matter in the universe.

You might think that physicists would be overjoyed at the result. But there was a lot of scepticism. "Extraordinary results require extraordinary evidence," particle physicists would tell me. They simply didn't believe that DAMA had found the leading candidate for dark matter, particles called WIMPs that barely interact with anything.

Part of the reason is that the LIBRA and DAMA teams have always taken a different approach to finding dark matter from other experiments. They assume that the amount of dark matter flooding past our planet depends on the Earth's direction of motion, so it varies with the seasons. So the DAMA team looked for a seasonal variation in its array of sodium iodide detectors. Although the signal was small, it varied just as predicted.

But the original DAMA experiment suffered from much more background noise than other competing experiments - which have not detected dark matter. So physicists assumed that something else must be causing the seasonal variation in the DAMA detector. Perhaps the temperature of the lab was fluctuating, or the detector was picking up the decay of radioisotopes in groundwater which ebbed and flowed with the seasons. Bernabei has always stood by the DAMA result and insisted that the team checked everything they could think of.

Fast forward to today and hunting for dark matter has become extremely popular. There are now around 30 dark matter experiments around the world looking for WIMPS using crystals chilled to a few degrees above absolute zero, giant tanks of ultrapure liquid xenon and even bubble chambers. By using ultrapure materials, freezing them and shielding the detectors from cosmic rays or other radiation, these experiments simply wait for a passing WIMP to leave a signal rather than look for messy seasonal variations.

Several of them have ruled out WIMPs with the properties that DAMA claimed, most recently the COUPP experiment at Fermilab near Chicago. But the DAMA result remains a thorn in the side of dark matter hunters, which is why I think it was right to repeat the experiment using improved technology.

A physicist close to the new LIBRA collaboration says that Bernabei will announce a positive signal for dark matter. If that's true, I reckon the LIBRA result could simply spark another round of controversy, rather than silence the sceptics.

Some scientists might be horrified that I and other bloggers are spreading rumours and speculation about Bernabei's talk. But the beauty of conferences is that you can pick up on the very latest ideas as they emerge. Dark matter research is reaching fever pitch and we should all be able to share the excitement.

Original here

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