DarkSide-20k: A 20 tonne two-phase LAr TPC for direct dark matter detection at LNGS

Building on the successful experience in operating the DarkSide-50 detector, the DarkSide Collaboration is going to construct DarkSide-20k, a direct WIMP search detector using a two-phase Liquid Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 t (20 t). The DarkSide-20k LAr TPC will be deployed within a shield/veto with a spherical Liquid Scintillator Veto (LSV) inside a cylindrical Water Cherenkov Veto (WCV). Operation of DarkSide-50 demonstrated a major reduction in the dominant 39Ar background when using argon extracted from an underground source, before applying pulse shape analysis. Data from DarkSide-50, in combination with MC simulation and analytical modeling, shows that a rejection factor for discrimination between electron and nuclear recoils of >3 × 109 is achievable. This, along with the use of the veto system, is the key to unlocking the path to large LAr TPC detector masses, while maintaining an “instrumental background-free” experiment, an experiment in which less than <0.1 events (other than ν-induced nuclear recoils) is expected to occur within the WIMP search region during the planned exposure. DarkSide-20k will have ultra-low backgrounds than can be measured in situ. This will give sensitivity to WIMP-nucleon cross sections of 1.2 × 10−47 cm2 (1.1 × 10−46 cm2 ) for WIMPs of 1 TeV/c2 (10 TeV/c2) mass, to be achieved during a 5 yr run producing an exposure of 100 t yr free from any instrumental background. DarkSide-20k could then extend its operation to a decade, increasing the exposure to 200 t yr, reaching a sensitivity of 7.4 × 10−48 cm2 (6.9 × 10−47 cm2) for WIMPs of 1 TeV/c2 (10 TeV/c2) mass.