Welcome to the DarkSide
The DarkSide program is a phased direct detection dark matter experiment. DarkSide focuses on a search for weakly interacting massive particles (WIMPs), a strong candidate for dark matter, with a mass of 100 GeV/c2. The program began with DarkSide-10 in 2010 at Princeton University. The next phase, DarkSide-50 is the first in the program with the ability for physics reach and has been operational since 2013. DarkSide-50 consists of three nested detectors: the Water Cherenkov Muon veto (WCV), the liquid scintillator neutron veto (LSV), and the dual-phase liquid argon time projection chamber (LAr TPC). DarkSide-50 is located underground at at a depth of 3800 meters water equivalent at the Laboratori Nazionali del Gran Sasso in Hall C. The WCV acts as a passive shield against external backgrounds. The LSV is an active neutron veto. It is a 4 meter diameter stainless steel sphere which resides inside the WCV and it is filled with 30 tonnes borated liquid scintillator. The LSV is monitored by 110 photomultiplier tubes (PMTs). Neutrons interact as one would expect a dark matter particle to interact; therefore, it is important to be able to tell when a neutron has entered the detector. The LSV is efficient at detecting radiogenic neutron captures at the level of at least 99.1%.
The LAr TPC is filled with 150 kg liquid argon total, with an active volume of 50 kg. Initially, DarkSide-50 was filled with atmospheric argon. However, atmospheric argon contains argon-39, a beta emitter. The presence of argon-39 would limit the sensitivity of the detector. Therefore, in the spring of 2015, DarkSide-50 was drained of atmospheric argon and then re-filled with argon sourced from underground. Underground argon is depleted in argon-39 due to the half-life of argon-39 (269 years) and the long time the argon has spent undergound. The LAr TPC has a total of 38 PMTs, 19 on the top and 19 on the bottom of the TPC arranged in a hexagonal pattern. When a particle enters the liquid argon, it will recoil off the argon atom producing scintillation. This first scintillation signal in the liquid is called S1. Electrons are then drifted, via a uniform electric field, towards the gaseous argon where they encounter a stronger electric field. This stronger electric field accelerates the electrons into the gaseous argon where they produce a secondary scintiallation signal, S2. The relationship between S1 and S2 is crucial in understanding the energy scale of the detector and how the detector responds to various interactions. The ratio of S2 to S1, while critically important to liquid xenon detectors, is used as a cross-check for liquid argon detectors as the rejection power of S1 alone is sufficient for WIMP searches. The current limit set by DarkSide-50 is 1.14 x 10-44 cm2 for a mass of 100 GeV/c2.
DarkSide-20k will be the next generation of the program. DarkSide-20k will be a 30 tonne (20 tonne fiducial) liquid argon TPC modeled after DarkSide-50 with a veto that can measure the ultra-low backgrounds in-situ. The detector will have a sensitivity of 1.2 x 10-47 cm2 for a WIMP mass of 1 TeV/c2 to be achieved during a 5 year background-free run. It is currently in the planning/building stage.
