About
The Global Argon Dark Matter collaboration combines the joint efforts of the DarkSide-50, miniCLEAN, ArDM and DEAP-3600 experiments to build DarkSide-20k.
The evidence for “dark matter” comes from astrophysical measurements of galaxies, large-scale galaxy clusters, and cosmic microwave background photons, which indicate that the majority of matter in the universe is not like the matter we are made of, and does not interact with light. Understanding the nature of dark matter is of fundamental importance to cosmology, astrophysics, and elementary particle physics.
Dark Matter Particles
Schematic representation of rotating disc galaxies in the distant Universe and the present day. Observations with ESO’s Very Large Telescope suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter. As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe. Their rotations curves, rather than being flat, drop with increasing radius.
One leading explanation is that dark matter is comprised of as-yet-undiscovered Weakly Interacting Massive Particles (WIMPs) formed in the early universe and subsequently gravitationally clustered in association with the baryonic matter we are made of.
In this scenario, galaxies like ours are submersed in a “halo” of dark matter which surrounds the standard luminous matter that we observe everyday with telescopes and antennas. As we rotate together with the Sun around the center of our Galaxy, we are hit by an apparent “wind” of WIMPs, too feebly interacting with standard matter to be sensed or seen.
In principle, WIMPs could be detected in terrestrial experiments through their collisions with ordinary atoms, producing observable low-energy (<100 keV) recoils in sensitive detectors. The predicted collision rates are extremely small and require ultra-low background experiments with large (1–100 tonne) target masses, located in deep underground sites to eliminate sources of background interactions that may mimic the tiny dark matter interaction signal.
Dual-phase time project chamber detector schematic
Direct Detection
DarkSide aims to detect rare dark matter interactions with argon atoms in dual-phase time projection chamber detectors (TPCs).
When a dark matter particle scatters with an argon atom, the struck atom recoils through the detector producing heat and exciting or ionizing atoms along the recoil’s path. Dual-phase TPCs measure both the scintillation light from de-excitation, and the ionization generated by recoiling nuclei. Most of the TPC is filled with liquid argon, with a small gaseous argon pocket at the top. An electric field is applied across the liquid to drift ionization charge towards the gas pocket, where it is extracted, proportionally amplified, and converted into light. This enables measurement of both the scintillation and ionization using light sensors like photomultiplier tubes (PMTs) or silicon photomultipliers (SiPMs),
Noble elements like argon are ideal targets to investigate WIMPs because they are stable, chemically inert, produce a large scintillation response to a tiny energy deposition, and can be readily purified into multi-tonne scale detectors.
Why Argon?
Direct dark matter detection in large dual-phase TPCs needs a target that is transparent to its own scintillation light, and relatively easy to extract and purify from radioactive contaminants. These impurities—emitting electrons, alpha particles, gamma rays, or neutrinos—could otherwise obscure a rare and tiny dark matter signal.
Argon stands out as the only noble element that enables part-per-billion discrimination between nuclear recoils—from potential WIMP interactions or neutron scattering—and electron recoils, which arise from trace-level radioactivity in the detector materials. This is because of argon’s chemistry: the timing distribution of the scintillation light resulting from a particle interaction in the TPC has a very different fraction of prompt light, in the first ~100 ns, relative to the full light pulse for electron vs. nuclear recoils. This prompt fraction can distinguish electron from nuclear recoil signals with extraordinary precision.
This powerful pulse shape discrimination is key to suppressing backgrounds and paves the way for an instrumental background-free WIMP search in DarkSide-20k.
How does pulse shape discrimination look like in DarkSide-50 data? This figure shows the fraction of prompt scintillation light (f90) vs. the total amount of scintillation light, S1, evaluated as total photoelectrons recorded by the photosensors, in DarkSide-50 data. The background interactions, mainly electron recoils from beta-decay of 39-Argon, sits at f90 of ~0.3, as only 30 percent of the scintillation light in these events is released in the first 90 ns. On the other hand, WIMPs, producing nuclear recoils, would have f90 centered at ~0.7, as shown in the violet shaded region. Figure from Phys. Rev. D 98, 102006 (2018).
About
The DarkSide Experiments
Detectors of the DarkSide program have pioneered innovative techniques in the search for dark matter, including the use of argon from underground gas wells rather than atmospheric sources to drastically lower the radioactive 39Ar background; an active neutron veto to strongly suppress neutron backgrounds; and the large-scale of use ultra-low background silicon photomultiplier sensors.
DarkSide-50, the first physics detector of the DarkSide program, was a 50 kg liquid argon TPC and produced its first WIMP search results using argon from the atmosphere in December 2014. In October 2015, DarkSide-50 produced the first ever WIMP search results using low-radioactivity underground argon. The Global Argon Dark Matter Collaboration formed in 2017, bringing together the expertise of all extant dark matter searches using liquid argon, including DEAP-3600 which operates at 3.6 tonne argon detector in an ultra-pure acrylic cryostat at SNOLAB. DEAP-3600 measured pulse shape discrimination at the part-per-billion level in atmospheric liquid argon, demonstrating rejection of more 39Ar events than DarkSide-20k will accumulate in a decade of operations with its underground argon target.
The Collaboration’s current effort is the DarkSide-20k experiment, which is now under construction at LNGS.
Schematic of DarkSide-20k infrastructure, under construction in Hall C of the LNGS underground laboratory in Italy, which is 1400m below the surface, inside the Gran Sasso massif. The cryostat that keeps the argon at 87K where it is liquid is shown in red, with its associated cryogenics infrastructure at left.
Working Principle
When dark matter collides with an argon atom in DarkSide-20k’s dual-phase TPC detector, the collision causes the struck atom to recoil. The recoil produces short tracks of ionized and metastable excited argon atoms along its path, after which a sequence of reactions occur that involve the recombination of electron-ion pairs and the formation of short-lived excited diatomic argon “molecules” termed dimers. These dimers then decay, emitting argon’s characteristic 128 nm scintillation light. But not all ionization electrons recombine with the ionized nuclei – some free electrons remain, which are drifted up through the LAr by an applied electric field to the gaseous argon region.
There are key differences in the response of LAr to low ionization-density events (such as β or γ interactions from radioactivity in detector materials) compared to heavily-ionizing nuclear recoil events (such as from dark matter scattering, or from neutron interactions). Low-density ionization leads to less recombination of the ionized electrons, and therefore, more free electrons than a nuclear recoil track of the same total energy. The ratio of ionization to scintillation thus allows a method for particle identification – those due to electron recoils versus those due to nuclear recoils.
The difference in ionization density also produces a significant difference in the time profile of the scintillation light for low- vs. high-ionization-density events. Argon scintillation light is emitted from two nearly degenerate “molecular” states, a long-lived (τ ~ 1.6 μs) triplet state, and a short-lived (τ ~ 6 ns) singlet state. The long-lived state is found to be non-radiatively quenched in tracks with high ionization-density. Thus, electron recoils have a longer scintillation duration, compared to nuclear recoils. This is a significant effect, and is the working principle behind “pulse shape discrimination,” which provides argon’s uniquely powerful background rejection technique.
DarkSide-20k exploits these powerful background suppression techniques in the dual-phase LAr time projection chamber by detecting the scintillation emission vs. time (termed the “S1” pulse) with silicon photomultiplier (SiPM) sensors. The secondary signal produced in the gas phase from drifted ionization charge converted into electroluminescence (termed the “S2” pulse) is also detected using SiPMs. DarkSide-20k’s SiPMs are most efficient at detecting light in the visible wavelength range, and so the detector surfaces are covered with a wavelength-shifting thin film (TPB) to convert argon’s native 128 nm light into 420 nm light. The position of the interaction that produced a recoil is reconstructed from the pattern of detected light, and the time difference between the S1 and S2 pulses. The energy of the recoil is reconstructed from the amount of detected light.
Why There Is An
Underground Detector
Cosmic rays striking the atmosphere produce a rain of particles at ground level. Putting dark matter experiments deep underground reduces the incident flux of this cosmic radiation.
DarkSide-50 was deployed underground at LNGS, 1400 m below surface, inside the Gran Sasso massif. Due to the extremely low expected event rate of dark matter – less than 1 event per kilogram per year – dark matter detection is highly challenging and the absence of distinctive features, such as knees or peaks, in the expected energy spectrum of the DM-nucleon interaction makes it even more difficult to identify a possible positive signal, further emphasizing the need to minimize background events. In order to minimize background events, it is a natural choice to deploy these experiments underground, where the cosmic ray background is reduced by several orders of magnitude, and to implement a shielding system to further reduce the number of events that are caused by external sources.
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The DarkSide Collaboration aims to unveil the nature of dark matter through liquid argon detector technology and innovative underground experiments. It is located at the Laboratori Nazional del Gran Sasso in L’Aquila, Italy.