A new look at dark matter


The DarkSide collaboration announced on February 21, 2018, at the “UCLA Dark Matter 2018” conference in Los Angeles, the best exclusion limit in the search for low-mass dark matter. This result was achieved with the DarkSide-50 detector, which uses a very weak radioactive argon target and is installed at LNGS (Gran Sasso Underground Laboratory, Italy). The contributions of the research teams of the APC (Astroparticle and Cosmology Laboratory, CNRS / Paris Diderot University / CEA / Paris Observatory) and LPNHE (Laboratory of Nuclear Physics and High Energy, CNRS / University Pierre and Marie Curie / University Paris Diderot), part of the DarkSide collaboration, have been decisive in achieving this success. The calibration experiment “ARIS” carried out on the ALTO site of the IPNO (Institute of Nuclear Physics of Orsay, CNRS / University Paris-Sud) was just as crucial.

In France, DarkSide is supported by IN2P3, LabEx UnivEarthS and Sorbonne Paris Cité University, which welcomed Cristiano Galbiati as a visiting professor.

A wide range of astronomical observations has shown that visible stars and gases in all galaxies, including ours, are immersed in a cloud of non-luminous matter, dark matter, much more abundant than ordinary matter. One of the main challenges of astroparticle physics is to directly detect the dark matter particles, the Weakly Interacting Massive Particles (WIMPs) through their possible interactions with ordinary matter.

To achieve this objective, direct dark matter detection experiments must be installed in an underground environment: the rock cover, acting as a shield, helps to stop “ordinary” cosmic particles and minimize their impact on the observation of rare phenomena. Indeed, the interaction rate of dark matter with ordinary matter is not only very low, but the energy released during these interactions is also minimal. For these reasons, the experiments use ever-evolving technologies to reach the high levels of sensitivity needed to enable a discovery that opens the way to the exploration of new laws of physics.

In this context, the DarkSide collaboration, which brings together research institutes from eight countries (Brazil, China, Spain, the United States, France, Italy, Poland and Russia) is at the forefront.


The Detector Darkside-50


A first detector, Darkside-50, is currently in the data acquisition phase. It is a time projection chamber (TPC) containing 150 kg of argon, of which 50 kg constitute the active mass. This argon, extracted from very deep wells in Colorado is characterized by its minimal content of radioactivity. These properties, combined with a very powerful data analysis technique, allow a cancellation of the background noise induced by natural radioactivity and, consequently, a reliable demonstration of the possible interactions of the dark matter.

The DarkSide-50 detector: the time-projection chamber, filled with liquid argon, is surrounded by an active liquid scintillator shield (in the sphere), all installed in a cylinder containing 1000 m3 of water. © Darkside Collaboration

DarkSide-50 is installed at the LNGS in Italy, 120 km from Rome, at a depth of 1400 meters to reduce a million times the cosmic rays. DarkSide-50 is also protected by a double shielding consisting of 30 tons of liquid scintillator and 1000 tons of ultrapure water, used to suppress the residual cosmic radiation and radioactivity of the rocks surrounding the laboratory.

The scientific results presented by the DarkSide collaboration after 570 days of data are divided along two axes.

On the one hand, for the search for large mass WIMPs (> 50 GeV / c2) 1, the scintillation and ionization signals of liquid argon have been exploited and no event has been observed in the region where the signal of dark matter is expected. The collaboration thus demonstrated the detector’s discrimination power for natural radioactivity signals, which is a very promising validation of the underlying technology. In particular, the discrimination between natural radioactivity and nuclear retreat is a strong confirmation of the capabilities of liquid argon technology.


On the other hand, the collaboration used the single ionization signal to explore weaker energies, where lower mass WIMPs (<10 GeV / c2) are expected. The absence of the scintillation signal does not allow to eliminate the background noise, the key to success was the extremely accurate modeling of the noise and the response of the detector. The result obtained with DarkSide-50 thus made it possible to extend the exclusion zone by an order of magnitude to WIMPs with a mass of less than 5 GeV / c2.

 DarkSide-50. © Collaboration Darkside

A fundamental contribution to this result comes from the Argon Response Ionization and Scintillation (ARIS) experiment which allowed detailed characterization of the response of liquid argon. The ARIS experiment, which uses a neutron beam, was carried out in the ALTO laboratory (Orsay) under the guidance of the French teams of APC and LPNHE in collaboration with IPNO. Accurate modeling of detector response and background noise was the result of several years of detailed Monte Carlo simulation of the DarkSide Family detectors developed by the APC and LPNHE teams.

The results of DarkSide-50 are very promising for the realization of a successful program of discovery of dark matter with liquid argon detectors. “This exciting result dramatically expands the physical reach of DarkSide’s technology, originally developed for dark matter research in the field of large masses,” said Cristiano Galbiati, professor at Princeton University and spokesperson for the collaboration. “This is the best way to launch the DarkSide-20k detector, whose construction has just been approved and will have a target nearly 1000 times larger than DarkSide-50”.


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