EDELWEISS : tracking the WIMPs to find the trail of dark matter

Paris, May 31, 2002

CNRS/CEA Press Release

The EDELWEISS experiment, gathering a collaboration of seven French teams of physicists and astrophysicists, is searching for Weakly Interacting Massive Particles, or WIMPs. These particles could explain in part the nature of Dark Matter which contributes for 99% of the Universe. On May 29th, at the "Neutrino 2002" conference in Munich, researchers from the French CEA and CNRS institutes have presented two important results: EDELWEISS is able, for the first time, to test a significant domain of supersymmetry with important precision, and contradict the results presented in February 2000 by the DAMA Italian experiment, which had reported the observation of WIMPs with a mass 60 times the proton mass.

Luminous matter only contributes for one per cent to the density of the Universe, and ordinary matter, made of protons and neutrons, represents at most 5% of this total density. The nature of the very large fraction of missing mass, named Dark Matter, is completely unknown and its identification represents one of the major questions of contemporary physics. A large part of this Dark Matter could appear under the form of very massive elementary particles, called WIMPs, surrounding every galaxy. Supersymmetric theories (SUSY), unifying the four fundamental interactions* predict the existence, not yet verified, of these massive particles. But the direct observation of their existence, through the detection of their interactions with ordinary matter, is extremely difficult. In fact, their interaction rate, whose range can be estimated from cosmological data and accelerator-based experiments, is expected to be extremely small: for one kilogram of detector, the most favorable interaction rate is predicted to be one interaction per day, and is probably much less. WIMPs are therefore even more difficult to catch than neutrinos, already interacting very weakly with matter.

In this search, the DAMA Italian experiment, set in the Gran Sasso Underground laboratory, near Rome, has published in 2000 results supporting the existence of a WIMP of mass about sixty times the proton mass, with an interaction rate approximately one interaction per day and per kg of detector. Now, EDELWEISS, set in the Fréjus Underground Laboratory, under the Alps, is the first experiment to explore a significant fraction of supersymmetric theories compatible with experiments realized at the large CERN accelerator, LEP. With its sensitivity, EDELWEISS is able the exclude the whole domain corresponding to the WIMP observed by the DAMA experiment, assuming that this WIMP is really a SUSY particle interacting with matter in the standard way.

During the forthcoming year, the sensitivity of EDELWEISS should increase by a further factor 5. A more ambitious version of the experiment, EDELWEISS-II, is presently assembled and tested in Lyon and Grenoble, and will be operated in Fréjus at the end of 2003. EDELWEISS-II will be able to accommodate more than 100 detectors (instead of 3 presently) and will increase its sensitivity by a factor 100. With this experiment, the predictions of a large fraction of SUSY theories will be tested, and the first clear interactions of WIMPs will, hopefully, be detected.

* The four fundamental forces govern interactions between particles of matter : the electromagnetic force, the weak nuclear force, responsible of radioactivity, the strong nuclear force which maintains together elementary particles inside composite particles and nuclei, and the gravitation.

Press contacts :

CNRS :
Geneviève Edelheit
Tel.: 33 1 44 96 47 60

CEA :
Pascal Newton
Tel.: 33 1 40 56 20 97
Alexandra Bender
Tel.: 33 1 40 56 17 16