The Dark Matter problem
A variety of astrophysical observations indicate that 83% of the matter in the Universe is non-baryonic and non-luminous and its nature is one of the fundamental puzzles in astrophysics today. An attractive solution for the so-called “Dark Matter” (DM) problem can be given by the particle physics in the form of a massive (~100 GeV) weakly-interacting neutral particle (WIMP) foreseen in the super-symmetric extension of the Standard Model. If such particle exists, it could have been produced shortly after the Big Bang and eventually formed a relic gas in the present universe if sufficiently stable. The most promising way to detect WIMP interactions with matter is via their rare elastic scattering with the atomic nuclei through:
χ+(A,Z)at rest → χ+(A,Z)recoil
DM direct search requires the capability of measuring recoil energies in the region of a few tens of keV with negligible backgrounds. From the experimental point of view those constraints call for some massive (~ton scale) detector with an excellent rejection power of the natural sources of radiation. For example, assuming a “canonical” WIMP halo model and a mass of 100 GeV, a WIMP-proton cross-section of 10-44 cm2=10-8 pb would yield about 1 recoil event per day per ton above 30 keVr for an argon detector.
ArDM (Argon Dark Matter) is a particle physics experiment which consists of a ton-scale double-phase liquid argon (LAr) detector, aiming at measuring signals from WIMPs. The experimental setup consists of a cylindric dewar with 80 cm diameter sensitive volume, delimited by a reflector covered with wavelength shifter, and a 120 cm maximal drift length. In order to get a high enough target mass the noble gas argon is used in the liquid phase as target material. About 850 kg of ultra-clean liquid argon are contained in the sensitive volume. Since the boiling point of argon is at 87 K at normal pressure, the operation of the detector requires a cryogenic system. The setup is completed with a purification system and a polyethylene neutron shield. The experiment is currently installed and under comissioning in the Canfranc Underground Laboratory (LSC) in Huesca (Spain).
The 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 kinetic energy of these recoils is in the range of 1-100 keV. The advantage of having a double-phase detection technique lies on the fact that both scintillation light and ionization charge can be measured at the same time, providing a powerful discrimination method between nuclear recoils and electron recoils produced by background events. The ionization and scintillation signals are measured by two arrays of photomultiplier tubes placed on the top and bottom parts of the detector.