ArDM
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ArDM (Argon Dark Matter) is a particle physics experiment based on a liquid argon detector, aiming at measuring signals from WIMPs (Weakly Interacting Massive Particles), which probably constitute the Dark Matter in the universe. Elastic scattering of WIMPs from argon nuclei is measurable by observing free electrons from ionization and photons from scintillation, which are produced by the recoiling nucleus interacting with neighbouring atoms. The ionization and scintillation signals can be measured with dedicated readout techniques, which constitute a fundamental part of the detector.
In order to get a high enough target mass the noble gas argon is used in the liquid phase as target material. Since the boiling point of argon is at 87 K at normal pressure, the operation of the detector requires a cryogenic system.
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[edit] Experimental goals
The ArDM detector aims at directly detecting signals from WIMPs via elastic scattering from argon nuclei. During the scattering, a certain recoil energy - typically lying between 1 keV and 100 keV - is transferred from the WIMP to the argon nucleus.
It is not known how frequently a signal from WIMP-argon interaction can be expected. This rate depends on the underlying model describing the properties of the WIMP. One of the most popular candidates for a WIMP is the Lightest Supersymmetric Particle (LSP) or neutralino from supersymmetric theories. Its cross section with nucleons presumably lies between 10−12 pb and 10−6 pb, making WIMP-nucleon interactions a rare event. The total event rate can be increased by optimizing the target properties, for example by increasing the target mass. The ArDM detector is planned to contain approximately one ton of liquid argon. This target mass corresponds to an event rate of approximately 100 events per day at a cross section of 10−6 pb or 0.01 events per day at 10−10 pb.
Small event rates require a powerful background rejection. An important background comes from the presence of the unstable 39Ar isotope in natural argon liquefied from the atmosphere. 39Ar decays via beta disintegration with a halflife of 269 years and an endpoint of the beta spectrum at 565 keV. The ratio of ionization over scintillation produced by electron and gamma interactions with argon is different from the ratio produced by WIMP elastic scattering. The 39Ar background is therefore well distinguishable, if a precise determination of the ionization/scintillation ratio is achieved. As an alternative, the use of 39Ar-depleted argon procured from underground well gases is considered.
Neutrons emitted by detector components and by materials surrounding the detector interact with argon in the same way as WIMPs. The neutron background is therefore indistinguishable and has to be reduced as well as possible, as for example by carefully choosing the detector materials. Furthermore, an estimation or measurement of the remaining neutron flux is necessary.
The detector is planned to be run underground in order to avoid backgrounds induced by cosmic rays.
[edit] Construction status
The ArDM detector is currently (August 2006) assembled at CERN; Geneva, Switzerland. Separate tests have been carried out for the light readout, the charge readout and the high voltage supply devices. The mounting of the detector above ground is planned to take place in December 2006. In a first phase of running the detector above ground, the readout devices and the high voltage supply will be tested and tuned in order to assess the detector performance characteristics as its energy threshold, its resolution and its background rejection power.
In a second phase (end of 2007), the detector is planned to be moved underground. An EoI (Expression of Interest) has been submitted to the Canfranc Underground Laboratory in Spain. During the run underground it will hopefully be possible either to detect the WIMP or to confirm/place limits on the WIMP-nucleon cross section.
Beyond the one-ton version, the detector size can be increased without fundamentally changing its technology. A ten ton liquid argon detector is a thinkable expansion possibility for ArDM. Current experiments for Dark Matter detection at a mass scale of 1kg to 100 kg with negative results demonstrate the necessity of larger-mass experiments, if one wants to know what Dark Matter really is.
