Pictorial image showing, superimposed to an optical image, the spatial distributions of ordinary matter (pink) and the one assigned to dark matter (blue) estimated studying the merging of two clusters of galaxies (Bullet Cluster)

There is a wide range of astronomical evidence that the visible stars and gas in all galaxies, including our own, are immersed in a much larger cloud of non-luminous matter, typically a factor of 10 times greater in total mass. The existence of this “dark matter” is consistent with evidence from large-scale galaxy surveys and microwave background measurements, indicating that the majority of matter in the universe is non-baryonic. The nature of this non-baryonic component is still totally unknown, and the resolution of the “dark matter puzzle” is of fundamental importance to cosmology, astrophysics, and elementary particle physics.

A leading candidate explanation, motivated by supersymmetry theory, is that dark matter is comprised of as yet undiscovered Weakly Interacting Massive Particles (WIMPs) formed in the early universe and subsequently gravitationally clustered in association with baryonic matter. WIMPs could in principle be detected in terrestrial experiments through their collisions with ordinary nuclei, giving observable low-energy (<100 keV) nuclear recoils. The predicted low collision rates require ultra-low background detectors with large (0.1–10 ton) target masses, located in deep underground sites to eliminate neutron background from cosmic ray muons.

TPC design for DarkSide-50

Among a number of developing detector technologies, two-phase liquid argon time projection chambers (LAr TPCs), which detect scintillation light and ionization generated by recoiling nuclei, are particularly promising. The signal/background discrimination power and the attainable precision for determining 3-D event positions have been demonstrated in published results from members of the present collaboration.

We propose to develop and operate a serics of new liquid argon detectors for WIMP detection. The first detector in the program intended to make physics measurements will be DarkSide-50 (DS-50), with 50 kg active mass, which is scheduled to begin commissioning at LNGS during CY 2012.  The program is intended to progress to multi-ton detectors with high sensitivity for WIMP detection.

Detectors of the DarkSide program will use several innovative techniques to positively identify Dark Matter signals and to understand and suppress  background.  These techniques include the use of argon from underground rather than atmospheric sources, to drastically lower the radioactive 39-Ar background; an active neutron veto to strongly suppress neutron backgrounds; and comprehensive measures to control background sources in the detector and photosensors.