All current cosmological observations converge to support the existence of "dark matter", invisible but dominating the dynamics of the Universe at all scales. The nature of this "invisible mass" is completely unknown but the existence of WIMPs, for weakly interacting massive elementary particles, is a particularly interesting case. Its mass is estimated as 5 times greater than that of known matter and would be present in abundance in our galaxy, the Milky Way. This missing mass within the Universe is one of the major questions that physicists will attempt to answer in the 21st century.
Detectors installed in the cryostat at Modane underground laboratory (Credit: CNRS/IN2P3). Several extensions of the particle physics Standard Model, predict the existence of such particles with an exceedingly low interaction power with matter. These particles could flow safely through all the atoms of the Earth, with only a very small fraction of them interacting by chance with its material. Research of WIMPs are therefore performed underground as the detectors are well protected from other cosmic radiation while retaining all their chances to observe the passing of a WIMP. An Edelweiss bolometer (Credit: CEA/IRFU).
Direct detection experiments, such as Edelweiss, aim to discover these particles by measuring these tiny collisions in specially designed detectors. If the number of WIMPs passing through the Earth every second is commensurate with the amount of the dark matter that seems to fill our galaxy, and if the mass of the detectors is large enough to achieve an observable collision rate, then discovering the secrets of dark matter is at reach. A global race to detect WIMPs has been going on for several years, and some experiments are beginning to have a chance of discovering WIMPs.
The research teams forming the Edelweiss collaboration, located in the Modane underground laboratory, have worked since the 1990s to develop germanium detectors operated at ultra-low temperature (20 mK), able to separate the signal produced by the interaction of WIMPs from the background due to natural radioactivity. The Edelweiss-II experiment, running since 2006, originated as a French collaboration that has expanded to a European level with the inclusion of laboratories from Germany, Russia and the United Kingdom. >>
|  An important turning point was reached in 2007 with the development of a new electrode design that enables the efficient rejection of natural radioactivity occurring at the surface of the detectors, which previously mimicked the expected WIMP signal features. These 400 g detectors were tested successfully in 2008, and used for the first time in a WIMP search the following year. Over a 6-month period, 10 of these detectors demonstrated high levels of reliability and robustness, improving the sensitivity to WIMPs by a factor of 15 relative to the previous generation of detectors. The preliminary analysis of an additional six months of data confirms their effectiveness at the WIMP hunt, with a further factor of two increase in sensitivity. New prototypes with larger mass, improved sensors implantation and readout capabilities have been installed this month. They will have the capability of addressing new forms of backgrounds , with hints having appeared in the recent data, and ultimately yield a factor 10 increase of sensitivity in the coming future. With these breakthroughs, Edelweiss will be able to join the elite group of the few experiments around the world able to achieve sufficient sensitivity to discover WIMPs as predicted by theoretical models. This community of competing but complementary experiments will be needed to confirm any hints of this major discovery. Interestingly, the coming years may see concur two very different experimental endeavours: the production of WIMPs at the LHC, at CERN, and, deep under the Fréjus mountain, Edelweiss may show that these particles indeed populate our galaxy, the Milky Way.
Submitted by Gilles Gerbier (CEA / IRFU - France)
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New results from the Edelweiss experiment have just been reported. After one full year of operation, the new-generation germanium detectors confirmed their spectacular ability to discriminate the background...
