In the past few decades, astrophysical and cosmological observations have yielded very convincing evidence that the visible objects in the sky, such as planets, stars, galaxies and inter-galactic dust makes up a mere ~4% of the total energy density of the universe. We have essentially no idea what the remaining 96% is. The study of the velocity profile of stars as a function of their distance from the center of the galaxy they belong to was the first clue for the presence of an invisible, massive constituent of the universe, i.e. subject to gravity but with no electromagnetic interaction. More recent observation of gravitational lensing effects and remnants of the collision of clusters of galaxies allow us to quantify such unknown massive component at ~22% of the energy density of the universe. The remaining 74% is in the form of unknown, dark energy that seems to be responsible for the accelerated expansion if the universe. Once particles like neutrinos, now known to have tiny masses, were shown insufficient to account for the missing mass, another particle candidate was proposed in the form of WIMPs, Weakly Interacting Massive Particles. These hypothetical particles would be massive (and heavy), non-relativistic, and would only interact with the weak nuclear force (other than gravity). This makes them very hard to detect!

DarkSide (Depleted Argon cryogenic Scintillation and Ionization Detection) is an experimental collaboration for the detection of WIMPs. Its scientific program is planned in phases with increasing sensitivity. The first step is Darkside-50, a dual-phase, 50 kg depleted argon time projection chamber (TPC). WIMPs would be detected when they scatter off an argon nucleus in the bulk liquid argon target. The nucleus recoils leaving a short track of ionized argon. Molecular processes of electronic recombination produce a flash of scintillation light detected by arrays of photomultiplier tubes (PMTs). By applying an electric field, a fraction of the electrons are drifted to the anode grid, multiplied in the gas phase above the liquid argon and detected as a second signal. This dual readout allows us to distinguish nuclear recoils from other background events. Darkside-50 will be deployed at the Gran Sasso underground laboratories in Italy. The second phase of DarkSide is within a larger project called MAX (Multi-ton Argon and Xenon detectors), which also includes the XENON collaboration. The plan is to run dual-phase ton-sized depleted argon and xenon detectors side by side, to validate discovery claims with different targets.

This is a new research area for EGPA. Planned activities include:

• Participation in the design and assembly phase of the Darkside-50 detector
• Geant4 detector simulations
• Study of scintillation and suitable light detectors in liquid argon
• Calibration and data analysis
• Contribution to a 20 kg prototype currently being operated at Princeton

• 04/2010: DarkSide is featured in the Spring 2010 Physics Department newsletter