DarkSide-20k

DarkSide-20k is a tonne-scale liquid argon dark matter experiment under construction at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, designed to push the search for WIMP dark matter into the neutrino fog.

The question

Astrophysical and cosmological observations show that most of the matter in the universe is dark: it does not emit or absorb light, and it is not made of any particle in the Standard Model. Weakly interacting massive particles (WIMPs) remain one of the best-motivated candidates. If they exist, they should occasionally scatter off atomic nuclei in a detector on Earth, depositing a few keV of energy. Seeing that signal requires a very large target, a deep underground site, and near-total suppression of backgrounds from natural radioactivity.

The experiment

DarkSide-20k is a dual-phase liquid argon time projection chamber (TPC). A particle interaction in the liquid produces a prompt flash of scintillation light and frees ionization electrons, which are drifted to a thin gas layer at the top of the detector and converted into a second, delayed light signal. Together the two signals give the energy and the three-dimensional position of every event.

Several features make the design distinctive:

Pulse-shape discrimination. In argon, nuclear recoils (the expected dark matter signal) and electron recoils (the dominant background) produce scintillation light with very different time profiles. This allows electron-recoil backgrounds to be rejected with extraordinary efficiency.
Underground argon. Atmospheric argon contains the radioactive isotope ³⁹Ar. DarkSide-20k fills its TPC with 50 tonnes of low-radioactivity argon extracted from an underground source in southwestern Colorado, where ³⁹Ar is strongly depleted.
Silicon photomultipliers. Instead of photomultiplier tubes, the TPC is read out by two 10.5 m² optical planes of cryogenic silicon photomultipliers (SiPMs), with a further 5 m² instrumenting the veto detectors — the largest SiPM-based photodetection system built for a dark matter experiment.
Active vetoes. Instrumented argon volumes surrounding the TPC tag neutrons and cosmic-ray muons that could otherwise mimic a dark matter signal.

The goal is a search that is effectively background-free over the full exposure, with sensitivity to WIMP–nucleon cross sections reaching the level at which coherent scattering of astrophysical neutrinos becomes an irreducible background.

Light dark matter

Using the ionization signal alone, a liquid argon TPC is also a powerful probe of dark matter particles lighter than 10 GeV/c². Building on the approach demonstrated with DarkSide-50, DarkSide-20k is expected to improve on existing sensitivity to low-mass WIMPs and to sub-GeV particles scattering off electrons by at least an order of magnitude within its first year of data, and to reach the neutrino fog for WIMP masses around 5 GeV/c² with ten years of exposure.

Our focus

Within the collaboration, our group works on three fronts:

Nuclear recoil calibrations. We measure the response of liquid argon to nuclear recoils on a dedicated test bench, the input needed to turn a detector signal into a dark matter recoil energy.
Inner detector pre-integration and installation. We take part in assembling and testing the inner detector ahead of time and in installing it underground at LNGS.
Machine learning for data analysis. We apply ML/AI methods to the analysis of DarkSide-20k data.