Research |

DarkSide-20k

Fri, 03 Jan 2025 00:00:00 +0000

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.

Selected publications

F. Acerbi et al. (2024). DarkSide-20k sensitivity to light dark matter particles. Commun. Phys. 7, 422.

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F. Acerbi et al. (2025). Sensitivity to low-mass WIMPs with an improved liquid argon ionization response model within the DarkSide program. Phys. Rev. D 113, 123045.

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F. Acerbi et al. (2024). Quality assurance and quality control of the 26 m² SiPM production for the DarkSide-20k dark matter experiment. Eur. Phys. J. C 85, 534.

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F. Acerbi et al. (2025). Production, quality assurance and quality control of the SiPM Tiles for the DarkSide-20k Time Projection Chamber. Eur. Phys. J. C 85, 1334.

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Fabio Acerbi et al. (2026). The DarkSide-20k Data Acquisition System. JINST 21, P09017.

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Detector R&D with SiPMs

Wed, 01 Jan 2025 00:00:00 +0000

We develop and exploit low-threshold silicon detectors to extend the dark matter search into the sub-GeV mass regime, where conventional noble liquid detectors lose sensitivity.

Why silicon photomultipliers

Noble-liquid detectors see the world through light. A particle interaction in liquid argon or xenon produces a faint flash of ultraviolet scintillation, and the performance of the experiment — its energy threshold, its energy resolution, its ability to tell signal from background — depends on how many of those photons are detected.

Silicon photomultipliers (SiPMs) have become the photosensor of choice for the next generation of experiments. Compared with traditional photomultiplier tubes they offer single-photon sensitivity at cryogenic temperatures, lower intrinsic radioactivity, compact geometry that allows nearly full coverage of large detector surfaces, and low operating voltage.

Using them at the scale of tens of square meters raises its own questions, which have been a thread through our work:

Photon detection efficiency at the vacuum-ultraviolet wavelengths of xenon (175 nm) and argon (128 nm) scintillation.
Correlated noise and dark counts, which set the effective threshold and energy resolution of a large-area detector.
Reflectivity of the sensor surface, which determines how light propagates in a detector whose walls are covered in silicon.
Operation in real detector conditions — at cryogenic temperature, immersed in the noble liquid, and in strong electric fields.
Large-scale production, from wafer-level cryogenic testing to assembly into tiles and photodetector units with reliable quality control.

Toward lower thresholds

The lightest dark matter candidates deposit so little energy that only a handful of photons or electrons are produced. Reaching these signals requires sensors with single-quantum sensitivity and extremely low noise. We are interested in pushing silicon-based detectors in this direction to open up the sub-GeV dark matter mass range.

Selected publications

Fabio Acerbi et al. (2026). Construction and characterisation of the DarkSide-20k veto silicon photo-multiplier tiles. Eur. Phys. J. C 86, 1089.

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G. Gallina et al. (2022). Performance of novel VUV-sensitive Silicon Photo-Multipliers for nEXO. Eur. Phys. J. C 82, 1125.

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P. Nakarmi et al. (2019). Reflectivity and PDE of VUV4 Hamamatsu SiPMs in Liquid Xenon. JINST 15, P01019.

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X.L. Sun et al. (2018). Study of Silicon Photomultiplier Performance in External Electric Fields. JINST 13, T09006.

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Ako Jamil, T. Ziegler, P. Hufschmidt, G. Li, L. Lupin-Jimenez, T. Michel, I. Ostrovskiy, F. Retière, J. Schneider, M. Wagenpfeil et al. (2018). VUV-sensitive Silicon Photomultipliers for Xenon Scintillation Light Detection in nEXO. IEEE Trans. Nucl. Sci. 65, 2823.

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EXO-200 & nEXO

Sat, 01 Jan 2022 00:00:00 +0000

Earlier work on the search for neutrinoless double beta decay of ¹³⁶Xe with liquid xenon time projection chambers: the EXO-200 experiment and its tonne-scale successor, nEXO.

EXO-200

EXO-200 was a single-phase liquid xenon time projection chamber, filled with xenon enriched in the isotope ¹³⁶Xe, that operated at the Waste Isolation Pilot Plant (WIPP) in New Mexico until 2018. By recording both the scintillation light and the ionization charge of each event, it achieved the energy resolution and background rejection needed to search for the neutrinoless double beta decay of ¹³⁶Xe.

The analysis of the complete dataset, corresponding to a ¹³⁶Xe exposure of 234.1 kg yr, found no evidence for the decay and set a lower limit on its half-life of 3.5 × 10²⁵ years (90% C.L.). The same data have been used for a broad range of other searches and measurements, including Majoron-emitting decay modes, double beta decay of ¹³⁴Xe, nucleon decay, and MeV-scale dark matter, as well as detailed studies of the response of liquid xenon. A subset of the calibration data has been released publicly for machine-learning development and education.

nEXO

nEXO is the proposed next-generation experiment: a time projection chamber holding 5000 kg of liquid xenon enriched in ¹³⁶Xe, with a projected half-life sensitivity of 1.35 × 10²⁸ years after ten years of data taking — enough to cover the parameter space of the inverted neutrino mass ordering.

A central element of the design is the detection of xenon’s 175 nm scintillation light with vacuum-ultraviolet-sensitive silicon photomultipliers. Characterizing these sensors — their photon detection efficiency, reflectivity, correlated noise, and behavior in strong electric fields — was a major focus of this work and carries over directly into our current .

Selected publications

G. Anton et al. (2019). Search for Neutrinoless Double-β Decay with the Complete EXO-200 Dataset. Phys. Rev. Lett. 123, 161802.

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G. Adhikari et al. (2021). nEXO: neutrinoless double beta decay search beyond 10²⁸ year half-life sensitivity. J. Phys. G 49, 015104.

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C. Chambers et al. (2018). Imaging individual barium atoms in solid xenon for barium tagging in nEXO. Nature 569, 203.

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S. Al Kharusi et al. (2025). Search for Double Beta Decays of ¹³⁴Xe with EXO-200 Phase II. Phys. Rev. Lett. 136, 232502.

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S. Al Kharusi et al. (2026). EXO-200 Public Data Release for AI/ML Applications.

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