The ArDM (Argon Dark Matter) Experiment 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.
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, such as 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 undergoes beta decay with a halflife of 269 years and an endpoint of the beta spectrum at 565 keV. The ratio of ionization over scintillation from electron and gamma interactions is different than WIMP scattering produces. The 39Ar background is therefore well distinguishable, with a precise determination of the ionization/scintillation ratio. As an alternative, the use of depleted argon from underground wells is being 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.
The ArDM detector was assembled and tested at CERN in 2006. Above ground studies of the equipment and detector performance were performed before it was moved underground in 2012 in the Canfranc Underground Laboratory in Spain. It was filled with was commissioned and tested at room temperature. During the April 2013 run underground, the light yield was improved compared to suface conditions.
Future cold argon gas runs are planned as well as continued detector development. Liquid argon results are planned for 2014.
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 1 kg to 100 kg with negative results demonstrate the necessity of ton-scale experiments.
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