The ArDM direct Dark Matter search
The ArDM (Argon Dark Matter, CERN RE18, LSC Exp-08-2010-ArDM) project is a direct Dark Matter search experiment, based on a ton scale liquid argon Time Projection Chamber. A leading candidate for the Dark Matter in the universe is the so-called WIMP (Weakly Interacting Massive Particle). The detector is designed to observe the recoiling argon nucleus after a WIMP-nucleus elastic scattering process. The recoiling nucleus is exciting and ionizing neighboring atoms, converting part of its kinetic energy into scintillation photons and free electrons. Both signals, from the scintillation light and the ionization charge, are measured independently. The pulse shape of the scintillation light (relative intensity of the fast and slow component) together with the light/charge ratio yields a strong signature to distinguish heavily ionizing recoiling nuclei from electron recoils, i.e. gamma and beta background. The scintillation light is detected by photomultiplier tubes (PMTs). The ionization charge is drifted in a uniform electric field to the liquid surface and extracted into the vapor phase, where the secondary scintillation light is produced and measured with PMTs. In a later phase of the experiment, it is considered to measure the ionization charge with LEMs (Large Electron Multipliers), yielding a better position resolution.
The ArDM detector was first assembled and tested on surface at CERN. In February 2012, all the cryogenic and vacuum infrastructure was moved to the underground laboratory of Canfranc (Laboratorio Subterráneo de Canfranc, LSC) in the Spanish Pyrenees. The cryostat has been commissioned successfully, and the lateral neutron shield, made of 13 tons of polyethylene, is in place. The detector will be installed early in 2013 and first runs at LSC are foreseen soon after.