DarkSide-20k: A 20 Tonne Two-Phase LAr TPC for Direct Dark Matter Detection at LNGS

Abstract: 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 (LArTPC) with an active (fiducial) mass of 23 t (20 t). The DarkSide-20k LArTPC 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 $^{39}$Ar 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 $\gt3\times10^9$ is achievable. This, along with the use of the veto system, is the key to unlocking the path to large LArTPC detector masses, while maintaining an "instrumental background-free" experiment, an experiment in which less than 0.1 events (other than $\nu$-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 $1.2\times10^{-47}$ cm$^2$ ($1.1\times10^{-46}$ cm$^2$) for WIMPs of $1$ TeV$/c^2$ ($10$ TeV$/c^2$) 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 $7.4\times10^{-48}$ cm$^2$ ($6.9\times10^{-47}$ cm$^2$) for WIMPs of $1$ TeV$/c^2$ ($10$ TeV$/c^2$) mass.

• The paper demonstrates a breakthrough in dark matter detection by utilizing a 23-tonne liquid argon TPC with advanced dual-phase design and pulse shape discrimination.
• It employs ultra-pure underground argon and innovative SiPM sensors to dramatically reduce background noise and enhance spatial resolution.
• Rigorous Monte Carlo simulations and diverse calibration methods validate the detector’s performance, setting a new standard for future low-background experiments.

Analysis of the DarkSide-20k Project for Direct Dark Matter Detection

The DarkSide-20k project represents an ambitious endeavor to advance the direct detection of dark matter through the deployment of a Large Volume Liquid Argon Time Projection Chamber (LAr TPC). It is designed to achieve unprecedented sensitivity to Weakly Interacting Massive Particles (WIMPs) by utilizing a 23-tonne active mass LAr detector. The DarkSide-20k aims to build upon the successes of the DarkSide-50, leveraging new technologies and an improved theoretical framework.

Key Components and Technological Aspects

LAr TPC Design

The LAr TPC core operating principle involves detecting primary scintillation light (S1) and ionization electrons (S2) produced by particle interactions. The ionization electrons are drifted in an electric field, extracted into the gas phase, and accelerated, leading to secondary scintillation. This process captures both light and ionization signals, allowing precise spatial and energy resolution with this dual-phase approach. The design emphasizes a high level of background rejection through pulse shape discrimination (PSD) techniques, demonstrated effectively with the smaller DarkSide-50.

Background Mitigation

The primary design goal of having an "instrumental background-free" environment necessitates extensive background reduction measures. These include using ultra-pure materials, underground argon sources to minimize isotopic contamination (e.g., the troublesome ^39Ar isotope), and robust shielding structures. The introduction of a sophisticated liquid scintillator veto (LSV) system enhances the interception of neutron backgrounds, which could mimic WIMP signals.

Improved Sensor Technology

A crucial advancement lies in the implementation of Silicon Photomultipliers (SiPMs) over traditional photomultiplier tubes (PMTs). SiPMs offer higher quantum efficiency, better imaging capabilities, and lower intrinsic radioactivity, thus making them an excellent choice for increasing the effectiveness of pulse shape discrimination and spatial resolution. The decision to equip the LAr TPC with SiPM modules marks a significant leap in optimizing detector sensitivity.

Simulation and Calibration

Rigorous Monte Carlo simulations have been employed to predict detector performances, particularly concerning light collection efficiency and background expectations. Calibration efforts are diversified, with radioactive sources and neutron generators ensuring that the detector is optimally tuned to capture both electron and nuclear recoil events, crucial for validating any potential WIMP signal.

Project Scale and Timeline

The DarkSide-20k project anticipates a total exposure of 100 tonne-years over a five-year period, with the option to double this exposure in an extended run. Success with DarkSide-20k could pave the way for future, even larger-scale projects, like the proposed Argo experiment targeting the neutrino floor.

Summary of Impacts

The outcomes from DarkSide-20k could significantly narrow the parameter space for WIMP detection, thus providing potential breakthroughs in our understanding of dark matter. The use of depleted underground argon (UAr) and novel PSD techniques strengthens its position among leading dark matter search experiments. The development of SiPMs sets a technological precedent for future low-background particle physics experiments beyond dark matter (e.g., neutrino physics). Ultimately, successful operation and results could offer new insights into both theoretical physics beyond the Standard Model and astrophysical models of dark matter distribution in the universe.