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The
visible mass of stars and interstellar gases represents only 10% of
the mass of galaxies. WIMPs (weakly interacting massive particles)
may be the key to this missing mass mystery. Several CNRS laboratories,
including the "Institut de physique nucléaire de Lyon"
(Nuclear Physics Institute, Lyon) are contributing to the EDELWEISS
experiment to detect WIMPs.
Supersymmetric particle physics indicates that WIMPs could collide
with matter but that the probability of interaction is small. The
EDELWEISS detector, a germanium-crystal bolometer, is therefore implanted
in the "Laboratoire souterrain de Modane" (Modane Underground
Laboratory), 1,700 meters deep in rock for protection from cosmic
rays and external radioactivity. The g-ray noise from surrounding
materials is still too great. The detector was designed to distinguish
between signals from the two source types to alleviate WIMP signal
masking.
The recoil of a germanium nucleus in the bolometer when a WIMP collides
with it produces two kinds of signal: an ionization type, due to electron-pair
formation, and a heat type, reflecting increased crystal temperature.
When combined, they give the nucleus recoil energy. A discrimination
factor Q dependent on the ionization : heat ratio helps distinguish
the two types of interaction.
Initial results indicate that no WIMPs hit the detector during the
counting time of the experiment. Participants in the joint Chinese-Italian
experiment, DAMA (DArk MAtter), believe that they have detected some,
a result that differs from that of the American CDMS (Cryogenic Dark
Matter Search) experiment. EDELWEISS is to increase the counting time
to determine which of the two teams has the correct theory.
Further investigation will require more detectors and the forthcoming
EDELWEISS II will deploy about 100 detectors. Results from that and
experiments by other international teams using different methods should
give a better idea of the character of the hidden matter.
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