01:00 03 May 2010 by David Shiga
For years, scientists running the DAMA experiment beneath a mountain in Italy have reported a seasonal variation in the number of light flashes seen in their sodium iodide target. They claim the variability is due to changes in the speed of dark matter particles streaming through the laboratory as a result of Earth's orbital motion around the sun.
In December 2009, scientists also reported two possible dark matter particle impacts in an experiment called the Cryogenic Dark Matter Search II.
Picture not related to this article/Foto não relacionada com este artigo: Symmetrymagazine Org.
CDMS-II, located in a Minnesota mine, is designed to detect dark matter particles by looking at the energy released when a particle smashes into a nucleus of germanium or silicon. Though the two candidate detections were unexpected, the researchers said they could not rule out the possibility that they were caused by interference from charged particles called cosmic rays or the decay of trace quantities of radioactive elements in materials used in the experiment.
Now, a third experiment called Xenon 100, which its makers say is even more sensitive than the other two, has failed to detect the impact of any dark matter particles, casting doubt on the earlier results. "Dark matter particles continue to escape our instruments," says Xenon 100 spokesperson Elena Aprile of Columbia University in New York.
Sensitive detector
Like DAMA, Xenon 100 is located in the underground Gran Sasso National Laboratory in Italy. But instead of sodium iodide, it uses liquid xenon as a target for dark matter particles.
Xenon is one of the heaviest elements in the periodic table. With three times the density of water, liquid xenon has many atoms per litre, maximising the chances that a dark matter particle will collide with it. If such a collision occurred, it would produce a flash of light that the experiment's cameras would observe.
Xenon 100 collected data for 11 days in October and November 2009. A new analysis of the data submitted to Physical Review Letters, the Xenon 100 team reports that only uninteresting background radiation events were detected, with no sign of dark matter.
The power of the Xenon 100 detector means it should see dark matter events at a much higher rate than DAMA or CDMS-II, making up for its much shorter running time, says Xenon 100 team member Rafael Lang of Columbia University in New York.
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Read more here/Leia mais aqui: New Scientist
