Construction of the first underwater neutrino telescope has just been completed. Since early June, the last two detection lines of Antares have been probing the bottom of the Mediterranean for neutrinos of cosmic origin. There are now 12 detection lines aimed at observing these elementary particles, which provide insight into the most violent phenomena in the Universe.
The completed telescope rewards the efforts of the European Antares(1) collaboration, particularly CEA-Irfu, IN2P3-CNRS, INSU-CNRS, and Ifremer, which have contributed largely to this project.
Neutrinos are elementary particles without an electric charge that interact very weakly with matter. Unlike other particles, neutrinos can move through the Universe in a straight line without being blocked by matter or deviated by magnetic fields along the way. Neutrinos are therefore unique among cosmic messengers, helping astrophysicists observe and better understand certain objects where cataclysmic phenomena occur. They open a new window on the Universe... provided scientists are able to detect them, which is far from simple as they interact very little with matter. For this reason, a neutrino detector must be as big as possible to increase the chances of intercepting these particles.
Teams at CEA and CNRS took up this challenge in 1996, marking the beginning of the Antares(2) project. In February 2006, after a long phase focused on studying the marine environment, the first flexible detection line, some 400 metres high, was laid at a depth of 2500 metres, off the coast of Toulon, using Ifremer's expertise and equipment. There are now 12 lines anchored to the seabed, covering a surface area equivalent to 4 football fields. They are equipped with nearly 900 optical modules, the "eyes" of the telescope, designed and built by the Antares teams. Half the lines were assembled at CPPM (particle physics centre in Marseille, CNRS/Université de la Méditerranée), which serves as the support laboratory for the experiment. The remaining lines were assembled at the Institute of Research into the Fundamental Laws of the Universe (CEA Irfu, Saclay).
The Antares detector is shielded from the background of cosmic rays by 2000 metres of water. Its abyssal depth provides total darkness, disturbed only by the faint light from a few bioluminescent animals. The telescope's basic principle is to make Earth itself the neutrino target. While the Earth stops other particles, neutrinos can pass all the way through. Along the way, some of them will collide with the nucleus of an atom. A very rare occurrence statistically, this collision produces a muon, a charged particle similar to an electron that moves in the same direction as the original neutrino.
The muon can travel up to a dozen kilometres in the Earth's crust. Once it enters the water, it leaves a very faint trail of light in its wake. The upgoing trail left by the muon is what the optical modules of Antares detect. In this way, the telescope looks through the Earth to observe the sky of the southern hemisphere. This portion of the sky includes the Galactic Centre, the site of extremely violent phenomena.
Neutrinos: a new window on the Universe
By tracking cosmic neutrinos, the Antares telescope aims to expand knowledge in high-energy astronomy. In recent decades, astronomers have discovered several very high-energy photon sources (galaxies containing supermassive black holes, supernova remnants, gamma-ray bursts, etc.). The photons may arise from the interaction of ultra-high-energy protons, which constitute the cosmic rays that bombard the Earth.
These nuclear reactions also appear to produce cosmic neutrinos. Astroparticle physicists can't observe cataclysmic events by detecting photons or protons, since at very high energies, these particles may be blocked by matter, making it hard to observe them in the distant Universe. In contrast, high-energy neutrinos move through the universe in a straight line and thus provide direct insight into these extremely violent phenomena. Neutrino detection by Antares should provide astrophysicists with a unique source of information about these events and gradually give rise to a new map of the sky.
Antares could also be used to observe lower-energy neutrinos created by the accumulation of dark matter at the centre of the Sun or the Milky Way. For 70 years, the missing mass of the Universe (95% of its total mass) has been one of the central questions in cosmology. Some of this missing mass could be made up of elementary particles called weakly interacting massive particles – or WIMPs for short. Their existence is predicted by the physics theory referred to as "supersymmetry", which also predicts that they accumulate at the centre of massive objects like the Earth or the Sun. WIMPs are both particle and antiparticle. If they were to accumulate, they would eventually cancel each other out, producing a burst of energy and particles, including low-energy neutrinos.
Antares also offers a permanent, underwater infrastructure for multidisciplinary scientific study. It is already equipped with instruments, some of which are installed together on a dedicated thirteenth line: seismographs, instruments for measuring temperature and oxygen concentration, a camera on the look-out for deep-sea fauna, and so on. Used in cooperation with INSU laboratories (COM, GeoAzur), these instruments will help find answers to questions asked in other scientific fields, such as oceanography or climatology.
Although the detector's total deployment is still recent, physicists have already used data from the first lines deployed to identify several hundred neutrinos, produced by the interaction of cosmic rays, in the atmosphere on the other side of the world from the detector. A few neutrinos from a distant source in the Universe might well be hidden among them, and it is only by accumulating data that researchers will be able to flush them out. Neutrinos are so difficult to track that physicists are working on a much larger detector, on a kilometric scale, one which will throw this new window on the Universe wide open.
1. More than 150 researchers, engineers, and technicians in Germany, Spain, Italy, the Netherlands, Romania, Russia, and in the following French laboratories: Centre de physique des particules de Marseille (CNRS/Université de la Méditerranée); CEA/Irfu (Institute of Research into the Fundamental Laws of the Universe, Saclay centre); Groupe de recherche en physique des hautes énergies (Université de Haute-Alsace); Institut pluridisciplinaire Hubert Curien (CNRS/Université Louis Pasteur); Astroparticule et cosmologie (CNRS, CEA, Université Paris Diderot, Observatoire de Paris); Géosciences Azur (CNRS, IRD, Observatoire de la côte d'Azur, Université de Nice, Université Pierre et Marie Curie); Centre d'océanologie de Marseille (CNRS/Université de la Méditerranée); Laboratoire d'astrophysique de Marseille (CNRS/Université de Provence); and Ifremer (Toulon/La Seyne-sur-Mer centre, Brest centre).
2. The Antares project is funded by contributions from CEA (DSM/Irfu) CNRS, the regional councils of Alsace and Provence-Alpes-Côte d'Azur, the local council of the Var département, the municipal council of La Seyne-sur-Mer, the European Union, and five countries (Netherlands, Germany, Italy, Spain, and Russia).
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