How are black holes formed? These are the kinds of questions that the Deep Underground Neutrino Experiment (DUNE) project could help answer. The neutrino detector at the heart of this experiment is currently under construction in the United States under the supervision of a consortium of 35 countries, including Canada, bringing together 1,400 scientists and engineers from 200 institutions. Once operational, around 2032, DUNE will rely on a colossal infrastructure. First of all, the PIP-II, an underground neutrino gun, the most powerful on the planet, located at FermiLab, in Illinois. Then, 1,300 km away, at SanfordLab, in South Dakota, a huge neutrino detector buried 1.5 km underground. The excavation of 800,000 tonnes of earth and rock necessary for its installation was completed in August 2024, after 7 years of work.
How will neutrinos travel from the gun to the detector? Through the earth’s crust! No problem crossing matter when you are a neutrino. Nicknamed “ghost particles”, neutrinos (not to be confused with neutrons) do not interact with their environment. Thus, every second, our body is crossed as if nothing had happened by a hundred thousand billion neutrinos coming from space. This lack of interaction with matter is what makes them so difficult to study.
However, on very, very rare occasions, and randomly, a neutrino interacts with the atom it passes through. The “shock” then produces other particles which, in turn, cause the expulsion of electrons. These are electrically charged and are therefore easy to detect. Their direction and energy make it possible to reconstruct the portrait and trajectory of the neutrino in question. This is the basic operation of most neutrino detectors around the world. These are enormous for increasing the chance of capturing an interaction, and also for detecting the particles emitted during this interaction, since they can go in all directions.
Thus, the DUNE detector will include four modules, each composed of detection grids immersed in a basin measuring 66 mx 19 mx 18 m, containing 17,000 tonnes of liquid argon, the equivalent of the volume occupied by 160 double-decker buses! Argon has the advantage of ultimately “generating” a lot of electrons, while being inexpensive and easy to purify. But to keep it in a liquid state, it will have to be kept at -184°C, using a gigantic cryogenics system!
As if that wasn’t already complicated enough, why do we also have to build the DUNE detector underground? “Because otherwise it would also detect cosmic rays [composés à près de 90 % de protons venant du Soleil]. And that these detections would drown out those of neutrinos, answers Roxanne Guénette, professor of particle physics at the University of Manchester, in the United Kingdom, and involved in DUNE. The Earth’s crust blocks cosmic rays, but not neutrinos, which pass through. » Working with ghost particles sometimes has advantages!
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1.5 km underground, at FermiLab, this “cave” will house two detectors from the DUNE project.
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The ProtoDUNE is a prototype detector that has been tested at the European Laboratory for Particle Physics in Switzerland. The FermiLab detectors will be 20 times larger.
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A component of PIP-II under construction in a clean room. This is a particle gun which will be used to produce the neutrino beam.
New views on the Universe
The Quebec researcher is impatiently awaiting the entry into operation of the DUNE facilities. “By producing neutrinos ourselves at FermiLab, we will be able to study them better. In addition, the detector will also capture neutrinos coming from space, which will help us understand the astronomical phenomenon that produced them. » So, when a dying star collapses in on itself, it emits massive amounts of neutrinos. These could tell us more about the post-mortem fate of the star, for example, its transformation into a black hole!
More broadly, DUNE will perhaps make it possible to adapt the standard model of physics, that is to say the solid theoretical and mathematical edifice which describes the foundations of the Universe. This model predicts that neutrinos have no mass. However, experimental data shows that these particles have one. Should we throw the standard model in the trash? Probably not, but we’ll have to revise it. And for this, it will be necessary to precisely determine the mass of the neutrino, possibly using DUNE. Will our understanding of the Universe depend on an experiment carried out underground?
Photos : CERN ; Reidar Hahn/Fermilab
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