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.
Read more about this topic: Ar DM
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